Email Infrastructure Glossary

A DNS record type mapping a hostname to an IPv4 address. Used for sending hostnames and must match PTR records for rDNS alignment.

Comparing two email versions to determine which performs better. MailWizz supports A/B testing campaigns. Best practice: test one variable at a time with statistically significant samples.

Percentage of outbound messages accepted by receiving ISPs at SMTP layer. Measures delivery before inbox vs. spam classification. High acceptance rate does not guarantee inbox placement.

PowerMTA's per-message delivery log in CSV format recording every delivery attempt: timestamp, virtual MTA, recipient domain, status, SMTP response, and dsnDiag. Primary tool for deliverability analysis.

A subscriber who engaged (open or click) within a defined window, typically 30, 60, or 90 days. Active subscribers form the initial segment for IP warming.

Email authentication extension (RFC 8617) preserving SPF/DKIM results through forwarding. Allows receiving servers to see original authentication results even when alignment breaks.

Autonomous System Number — unique number assigned to a network. ISPs use ASN data to group IP blocks for reputation assessment.

A source of email that fails authentication for a domain — a third-party or legacy system sending without proper DKIM/SPF. DMARC aggregate reports identify gaps before enforcing policy.

Header added by receiving servers documenting SPF, DKIM, and DMARC results. Visible in original message source: Authentication-Results: mx.google.com; dkim=pass; spf=pass.

Automated email sequence triggered by subscriber actions. In MailWizz, time-based sequences attached to a list. Should be excluded from initial IP warming segments.

Network data transfer capacity. Production PowerMTA servers require 1 Gbps minimum. Individual bulk campaigns consume modest bandwidth (30KB average message size).

Standard allowing senders to display brand logos next to authenticated messages in Gmail, Yahoo, and Apple Mail. Requires DMARC at p=quarantine or p=reject and (for Gmail) a Verified Mark Certificate.

Database of IP addresses or domains identified as spam sources, used by ISPs to filter mail. Major blacklists: Spamhaus ZEN, Barracuda, Invaluement, SORBS. Cloud Server for Email monitors 50+ blacklists daily.

An email that cannot be delivered. Hard bounces (5xx) indicate permanent failure requiring suppression. Soft bounces (4xx) are temporary deferrals to retry.

Percentage of sent messages that bounce. Hard bounce target: below 0.5%. Warning threshold: above 2%. ISP action threshold: above 5%.

An IMAP mailbox receiving bounce notifications returned to the MAIL FROM address. MailWizz processes these to classify and suppress bounced addresses automatically.

Adding hard-bounced addresses to a suppression list to prevent future delivery attempts. Critical for maintaining bounce rate within ISP thresholds.

A single bulk email send to a list or segment in MailWizz. Statistics: delivery rate, open rate, click rate, bounce rate, unsubscribe rate.

US federal law governing commercial email. An opt-out regime allowing commercial email without consent, provided it meets content and opt-out requirements.

Canada's anti-spam law requiring express or implied consent before sending Commercial Electronic Messages. Penalties up to CAD$10M per violation for organizations.

Percentage of delivered emails with at least one link clicked. A stronger engagement signal than open rate since it's not inflated by Apple MPP.

A DNS record aliasing one hostname to another. Used for tracking domain configuration (track.yourdomain.com points to tracking server) and some DKIM setups.

A recipient marking an email as spam. Gmail action threshold: 0.10% complaint rate. FBL programs provide complaint data from Yahoo and Comcast.

Percentage of recipients marking a message as spam. Target: below 0.08% (Gmail). Action threshold: 0.10% (Gmail begins filtering). Measured via Postmaster Tools and FBL data.

Limiting concurrent SMTP connections to a recipient ISP via max-smtp-out in PowerMTA domain blocks. Prevents overwhelming ISP rate limits and triggering 421 deferrals.

Documentation proving subscriber gave valid consent: email, timestamp, consent language shown, and collection method. Required for GDPR and CASL compliance.

Spam detection based on email content. Modern ISPs weight engagement signals over content, but content quality still contributes to filtering decisions.

Enterprise email security (Proofpoint, Mimecast, Barracuda) filtering inbound email. More aggressive than consumer ISP filtering.

Clicks divided by opens. Measures content effectiveness for engaged readers independently of list quality variations.

A sender-owned subdomain for open/click tracking. Replaces the email platform's default tracking domain. Improves branding and supports authentication alignment.

Legally binding contract between data controller and processor under GDPR Article 28. Cloud Server for Email provides DPAs for all EU infrastructure clients.

GDPR rights including: access, rectification, erasure, restriction, portability, and right to object. MailWizz supports data export and deletion.

Facility housing server hardware. Cloud Server for Email operates from Tier III the EUn datacenters with EU data residency and connectivity to EU internet exchanges.

An IP address used exclusively by one sender. Gives full control over IP reputation. Required for high-volume bulk sending where reputation isolation is essential.

A physical server allocated exclusively to one client. Provides predictable I/O performance and network resources essential for production PowerMTA spool operations.

A temporary delivery failure (4xx SMTP response). PowerMTA retries per the retry-after schedule. High deferral rates from a specific ISP indicate reputation or rate-limiting concerns.

Percentage of delivery attempts resulting in 4xx deferrals. Target: below 5% per major ISP. Rising deferral rate is the primary early warning signal for IP reputation degradation.

The ability of email to reach recipients' inboxes. Depends on IP reputation, domain reputation, authentication, list quality, and engagement rates. Measured at inbox placement rate.

Email authentication standard (RFC 6376) for cryptographically signing outbound messages. Recipients verify the signature using the public key in DNS. Proves the message came from the claimed domain and was not modified in transit.

Periodically replacing DKIM signing key pairs — recommended annually at minimum. Rotation requires adding a new DNS selector, updating PowerMTA configuration, then waiting 48 hours before removing the old selector.

The name component of a DKIM DNS record identifying which key pair signed a message. Format: [selector]._domainkey.[domain]. Multiple selectors allow multiple keys to coexist simultaneously.

Email authentication framework (RFC 7489) building on SPF and DKIM to provide domain-level policy enforcement and reporting. Allows domain owners to specify how receivers handle unauthenticated messages.

XML report sent daily by receiving servers to the DMARC rua address. Contains IP-level authentication pass/fail data. Essential for identifying unauthorized senders before advancing DMARC policy.

The requirement that the Header From domain matches the domain authenticated by SPF (envelope sender) or DKIM (d= tag). Relaxed alignment allows organizational domain match; strict alignment requires exact match.

The DMARC percentage tag specifying what percentage of messages the policy applies to. pct=5 applies policy to 5% of messages — useful for graduated enforcement rollout.

The DMARC p= tag: p=none (report only), p=quarantine (send to spam), p=reject (reject message). Advance from p=none toward p=reject as you verify all legitimate sources authenticate correctly.

Local storage of DNS query results for the TTL duration. Pre-emptively lowering TTL before urgent changes reduces the cache propagation window.

Time for DNS changes to become visible globally. Depends on TTL. Most changes propagate within 15 minutes to 4 hours.

Time-to-live: how long resolvers cache a DNS record (in seconds). 3600 (1 hour) is typical for email authentication records. Reduce to 300 before DKIM key rotations.

DNS-based blacklist of spam-sending IPs or domains. Queried by mail servers. Major DNSBLs: Spamhaus ZEN, Barracuda, Invaluement, SORBS, UCEPROTECT.

PowerMTA configuration section specifying delivery parameters for a recipient domain. Contains max-smtp-out, max-msg-rate, retry-after, max-msg-per-conn. The primary per-ISP optimization mechanism.

The reputation of the sending domain at ISPs, distinct from IP reputation. Google Postmaster Tools reports domain reputation separately. Can survive IP changes if domain maintains clean sending history.

Subscription requiring email verification by clicking a confirmation link. Provides strong GDPR/CASL consent documentation. Reduces invalid addresses. Configured in MailWizz under List Settings.

The DSN diagnostic field in PowerMTA's accounting log containing the full SMTP response text from receiving servers. Used to diagnose specific ISP delivery problems.

The hostname PowerMTA announces in SMTP EHLO command. Must match the PTR/rDNS hostname for the sending IP. Mismatch causes delivery failures, particularly at European ISPs.

Being added to a DNS blacklist due to spam complaints, spam trap hits, or policy violations. Requires addressing the root cause before submitting a removal request.

Forging email headers to make email appear from a trusted source. DMARC enforcement (p=quarantine or p=reject) actively rejects spoofed messages failing authentication.

A reusable HTML email design in MailWizz with personalization tags. Well-structured HTML improves rendering across clients and reduces spam scoring.

Natural decline in email engagement over time. Most pronounced 6-18 months after subscription. Informs re-engagement campaign timing and suppression decisions.

ISP filtering based on recipient engagement history with a specific sender. Gmail uses per-recipient engagement data to customize filtering.

Dividing subscribers by engagement history. Segments: 30-day openers, 90-day, lapsed, inactive. Essential for deliverability management and IP warming.

The MAIL FROM address in the SMTP transaction. Receives bounce messages. Used for SPF alignment. Also called return path or bounce address.

EU Directive 2002/58/EC requiring prior consent for unsolicited commercial email to individuals. Being replaced by ePrivacy Regulation (not yet adopted as of 2026).

A company providing email marketing services. Shared ESPs (Mailchimp, SendGrid) pool IP reputation. Dedicated infrastructure (Cloud Server for Email) provides owned IP reputation and full control.

ISP service sending spam complaint notifications (ARF reports) to registered senders. Yahoo and Comcast provide FBLs. Gmail provides aggregate data via Postmaster Tools. PowerMTA processes FBL data via pipe delivery.

Network security controlling inbound/outbound traffic. Email servers: allow outbound port 25, inbound port 25, inbound port 587, SSH from specified IPs. Block all other inbound.

EU regulation governing personal data processing. Requires valid legal basis for marketing, data subject rights, data minimization, and security. Penalties up to €20M or 4% of global turnover.

Multi-factor classification combining IP reputation, domain reputation, content signals, and engagement history. Weighs engagement data most heavily.

Free Google tool (postmaster.google.com) providing domain and IP reputation data. Shows: domain reputation tier, IP reputation, spam rate, and authentication status. Essential daily monitoring.

A permanent email delivery failure (5xx SMTP response). Must be immediately suppressed. Causes: invalid address, non-existent domain, policy rejection.

Messages placed in a held state in PowerMTA, not being delivered. Released via pmta resume. May be held due to policy or as a management action.

Internet Message Access Protocol — email retrieval protocol. In bulk email, used for bounce server configuration: MailWizz connects to an IMAP mailbox to process bounce notifications.

CASL consent basis for existing business relationships (2-year limit), published business addresses, and business card exchanges. Expires without conversion to express consent.

DNS domain for IPv4 reverse DNS (PTR) lookups. PTR for IP 203.0.113.1 stored at 1.113.0.203.in-addr.arpa.

Whether a delivered email lands in inbox vs. spam folder. The ultimate deliverability metric. Affected by IP reputation, domain reputation, engagement history, and content quality.

Percentage of delivered emails landing in inbox vs. spam folder. Measured via seed list testing. Target: 90%+ inbox placement.

Reputation-based IP/domain blacklist used by many ISPs. Listings from sustained spam trap hits. Removal requires addressing underlying list quality issues.

Input/Output Operations Per Second. Critical for PowerMTA spool directory. NVMe SSDs: 500,000+ IOPS. Required for production deployments above 500,000 daily messages.

A group of sending IPs in PowerMTA configured for a specific traffic type. Separate pools for transactional, marketing, and cold email provide reputation isolation.

The trust level assigned to a sending IP by ISPs. Visible via Postmaster Tools (Gmail) and SNDS (Outlook). Factors: complaint rate, spam trap hits, blacklist status, sending consistency.

Gradually increasing volume from a new IP to establish positive ISP reputation. Requires 8-12 weeks sending to engaged subscribers before production volume.

Intelligent Platform Management Interface for remote server administration, power control, and console access independent of OS. Available on dedicated server plans.

Junk Mail Reporting Program — Microsoft's feedback loop for Outlook.com complaint data. Enrollment at sendersupport.olc.protection.outlook.com/jmrp/

GDPR legal basis under Article 6(1)(f) for processing without consent. Can apply to relevant B2B marketing. Requires documented balancing test.

Rate at which subscribers leave through unsubscribes, bounces, and complaint-driven suppression. High churn indicates engagement problems or excessive sending frequency.

Regularly cleaning subscriber lists to remove invalid addresses, hard bounces, and chronic non-openers. Maintains low bounce and complaint rates.

Email header (RFC 2369) enabling one-click unsubscribe from email clients. RFC 8058 variant enables POST-based one-click. Required by Google for bulk senders sending 5,000+ messages/day to Gmail.

Distributing sending traffic across multiple servers or IPs. PowerMTA distributes across IP pool members. MailWizz distributes across delivery servers using probability-based assignment.

The envelope sender address specified in SMTP. Used for SPF evaluation and bounce destination. For DMARC, MAIL FROM domain must match the Header From domain.

Actively-maintained self-hosted email marketing platform (PHP/MySQL). Provides list management, campaigns, autoresponders, bounce processing, and statistics. Integrates with PowerMTA for delivery.

MailWizz REST API for programmatic subscriber management, list operations, and campaign triggering. API keys generated in MailWizz backend. Enables CRM and e-commerce integrations.

Scheduled tasks running MailWizz background processes: campaign sending, bounce processing, autoresponders. Must run every 5 minutes. Misconfigured cron jobs are the primary cause of campaigns not sending.

User-defined subscriber data fields. Support GDPR compliance documentation (consent date, source), personalization, and engagement tracking for warming segmentation.

A configured SMTP connection in MailWizz for outbound campaigns. Supports multiple delivery servers with probability-based load distribution. Each has SMTP credentials and hourly quota.

Filtered subscriber subsets based on conditions. Engagement-based segments (30-day openers) are essential for IP warming and deliverability management.

Global suppression list preventing any suppressed address from receiving campaigns. Populated automatically from hard bounces, unsubscribes, and FBL complaints.

Open and click tracking replacing links with custom tracking domain URLs and embedding a 1x1 tracking pixel. Custom tracking domain configuration required for authentication alignment.

Domain block parameter specifying maximum messages per SMTP connection session. After this number the connection closes and a new one opens. Balances connection efficiency against ISP session limits.

Domain block parameter specifying maximum messages per hour to a recipient ISP per virtual MTA. Setting below the ISP throttle threshold prevents queue buildup from deferrals.

Domain block parameter specifying maximum concurrent SMTP connections to a recipient ISP per virtual MTA. Should match IP's reputation tier. Too low wastes throughput; too high causes 421 deferrals.

Business email filtering via Exchange Online Protection (EOP). More aggressive than Outlook.com consumer filtering.

Smart Network Data Services — Microsoft's per-IP reputation service showing IP status (Green/Yellow/Red). Essential daily monitoring for Outlook deliverability.

Multipurpose Internet Mail Extensions — standard defining multi-part email messages (HTML + plain text + attachments). Proper MIME structure improves rendering and reduces spam scoring.

Apple's feature pre-loading email images before the user opens the message. Inflates open rates for Apple Mail users. Click data remains reliable.

Software transferring email between servers via SMTP. PowerMTA is the commercial MTA standard for high-volume bulk email. Postfix and Exim are open-source alternatives.

DNS record specifying the mail server receiving email for a domain. PowerMTA's mx-rollup groups domain families sharing the same MX infrastructure into a single throttle bucket.

Directive grouping multiple receiving domains sharing the same MX infrastructure into a single throttle bucket. Example: gmail.com and googlemail.com share Gmail's MX and should share throttle limits.

Non-Volatile Memory Express SSD providing very high IOPS. Critical for PowerMTA spool directory performance at high message volumes.

Percentage of delivered emails with tracking pixel loaded. Note: Apple MPP inflates open rates by pre-loading images. Click rate is a more reliable engagement metric.

Process of unsubscribing. CAN-SPAM: 10 business days. GDPR and CASL: without undue delay (immediately in practice). MailWizz processes opt-outs instantly.

Microsoft Outlook filtering combining SmartScreen, SNDS IP data, and content analysis. SNDS status (Green/Yellow/Red) directly indicates filtering treatment.

Email sent only to recipients who explicitly opted in. Produces lowest complaint rates and highest deliverability.

CAN-SPAM requirement in every commercial email. Can be street address, P.O. Box, or registered address. Included in MailWizz email footer template.

PowerMTA's ability to deliver email to an external process rather than via SMTP. Used for FBL processing: complaint reports processed by a handler that suppresses the complaining address.

PowerMTA's management CLI. Key commands: pmta show status, pmta show queue, pmta show domain, pmta pause queue, pmta resume, pmta flush, pmta reload.

Traditional SMTP port for server-to-server email delivery. PowerMTA uses port 25 for outbound delivery. Many ISPs block outbound port 25 from residential IP ranges.

SMTP submission port for application-to-server message submission. Used for PowerMTA injection from MailWizz. Uses STARTTLS for encryption and SMTP AUTH for authentication.

Primary configuration at /etc/pmta/config. Contains virtual MTAs, domain blocks, smtp-pattern-list, logging config. Changes applied via pmta reload without losing the message queue.

Commercial license from Port25/Zeta Global required to run PowerMTA. Typically per-server, unlimited volume. Included in all Cloud Server for Email managed PowerMTA plans.

Required document explaining subscriber data collection, use, storage, and sharing. GDPR requires concise, transparent language. Recommended link in marketing email footers.

DNS record mapping an IP address to a hostname (reverse DNS). Required for all sending IPs. Must match PowerMTA's EHLO hostname, with A record resolving back to the same IP (forward-confirmed rDNS).

Collection of messages awaiting delivery in PowerMTA. Consistently growing queue indicates delivery rate below injection rate, requiring investigation.

Targeted campaign reactivating subscribers who have not engaged in 90-180 days. Non-responders should be suppressed to protect reputation.

Domain block parameter specifying minimum time between retry attempts for deferred messages. ISPs sometimes include Retry-After hints in 421 responses. Respecting retry timing is a positive reputation signal.

The Return-Path header recording the envelope sender address. Added by the final receiving server. Bounce messages sent to return path. Used for SPF alignment evaluation.

IETF standard for one-click unsubscribe via HTTP POST. Required by Google for bulk senders. The List-Unsubscribe-Post: List-Unsubscribe=One-Click header triggers a POST request in supporting email clients.

GDPR Article 17 right allowing deletion of personal data on request. MailWizz supports subscriber deletion for data erasure requests.

Regional Internet Registry for Europe, the Middle East, and Central Asia. Cloud Server for Email's IPs are RIPE NCC allocated, providing EU-origin addresses.

Overall trustworthiness of a sender as assessed by ISPs. Comprises IP reputation, domain reputation, engagement rates, complaint rates, and authentication compliance.

Security configuration reducing attack surface: SSH key-only auth, firewall rules, disable unnecessary services, software updates, fail2ban for brute force prevention.

Simple Mail Transfer Protocol (RFC 5321) — the internet standard for email transmission. Response codes: 2xx success, 4xx temporary failure, 5xx permanent failure.

SMTP authentication requiring credentials before message submission. Used for email submission (port 587). PowerMTA supports SMTP AUTH for injection from MailWizz.

Three-digit number from SMTP server indicating command result: 2xx success, 4xx temporary failure, 5xx permanent failure. Recorded in PowerMTA's accounting log dsnDiag field.

PowerMTA configuration section mapping SMTP response text patterns to bounce disposition codes (perm=hard bounce, temp=soft bounce). A comprehensive pattern list enables accurate bounce classification.

A temporary email delivery failure (4xx SMTP response). PowerMTA retries per retry-after schedule. Causes: mailbox temporarily full, server unavailable, rate limit exceeded.

Where ISPs deliver messages classified as likely spam. Consistent spam folder placement indicates IP or domain reputation problems.

An email address used to identify spammers. Pristine traps were never real addresses. Recycled traps were once valid but made inactive. Hitting spam traps causes blacklistings.

Content, header, or behavioral pattern triggering spam classification. Includes excessive caps, known spam phrases, image-only emails, and broken HTML.

Major anti-spam organization operating widely-used blacklists: SBL, XBL, PBL, DBL, and ZEN. Spamhaus listings are among the most impactful for email deliverability.

Email authentication standard (RFC 7208) specifying which IP addresses are authorized to send email for a domain. Published as a DNS TXT record. Receiving servers verify the connecting IP against the SPF record.

DMARC requirement that the SPF-authenticated domain (MAIL FROM) matches the Header From domain. Required for DMARC to pass on SPF evidence.

Reducing DNS lookups in an SPF record below the 10-lookup limit by replacing include: mechanisms with explicit ip4: entries. Prevents SPF PermError from exceeding lookup limits.

SPF evaluation error from syntax errors or exceeding the 10 DNS lookup limit. Treated as SPF failure by some receivers. Prevented by keeping SPF records within lookup limits.

A DNS TXT record specifying authorized sending IPs. Format: v=spf1 [mechanisms] [qualifier]. Common qualifiers: ~all (soft fail), -all (hard fail).

Disk location where PowerMTA stores queued messages. I/O performance is critical — NVMe SSD required for production deployments above 500,000 daily messages.

SMTP extension upgrading a plain connection to TLS encryption. PowerMTA uses opportunistic STARTTLS for outbound connections. Most major ISPs support TLS.

List of addresses that should never receive email. Includes hard bounces, unsubscribes, FBL complaints. Legal requirement under CAN-SPAM and GDPR.

ISP-applied limitation on message acceptance rate (421 responses). PowerMTA's max-msg-rate proactively limits send rate to stay within ISP thresholds.

Cryptographic protocol providing encrypted communication. Used via STARTTLS for SMTP and HTTPS for tracking domains. TLS 1.2 minimum, TLS 1.3 preferred.

DNS record storing arbitrary text data. Used for SPF, DKIM public keys, DMARC, and domain ownership verification. Multiple TXT records allowed; SPF requires exactly one v=spf1 record.

Required under CAN-SPAM, CASL, and GDPR. Must be prominent, functional, and honored promptly. RFC 8058 one-click unsubscribe required by Google for bulk senders.

Percentage of time a service is operational. Cloud Server for Email targets 99.5% availability measured over rolling 30-day windows per the SLA.

URI Blacklist checking URLs in email bodies against known spam domains. Production senders should verify tracking and unsubscribe domains are not URIBL-listed.

URL tracking parameters for Google Analytics attribution: utm_source, utm_medium, utm_campaign. Standard for email: utm_source=email&utm_medium=newsletter.

A named PowerMTA entity representing a sending IP address, organized into pools for traffic routing. Each VMTA has a specific source IP assignable to domain blocks.

Digital certificate required for BIMI brand logo display at Gmail. Issued by DigiCert or Entrust. Validates the certificate holder as the legitimate trademark owner for the BIMI logo.

A virtual machine sharing physical hardware with dedicated resource allocations. Suitable for low-volume testing. Production bulk email requires dedicated servers for predictable I/O.

The first email sent to a new subscriber. Typically has 50%+ open rates. Including welcome emails in warming sequences builds positive engagement signals.

Yahoo Mail filtering combining IP reputation, DKIM authentication (strictly enforced), engagement signals, and FBL complaint data.

Spamhaus's combined blacklist (SBL + XBL + PBL). Most widely queried blacklist in email infrastructure. A ZEN listing indicates the IP is a known spam source, exploited network node, or residential IP range.