Managed outbound SMTP relay powered by our SMTP relay. Authentication, per-ISP throttling, and delivery monitoring for any application stack.
An SMTP relay is a mail server that accepts outbound email from an application or another mail server and forwards it to the recipient's mail server. In production bulk email infrastructure, the relay layer separates the application (MailWizz, custom application, CRM) from the final delivery engine (our relay), enabling centralized control over authentication, throttling, routing, and monitoring across multiple sending applications.
Cloud Server for Email operates managed SMTP relay environments that serve as the intelligent delivery layer between your application and recipient ISPs. Every message injected through our relay infrastructure passes through our relay's per-ISP domain block configuration, DKIM signing, Intelligence Bounce™ bounce classification, and accounting log recording — regardless of which application injected it.
Applications can deliver email directly to recipient MX servers, but this requires each application to manage its own SMTP sessions, retry logic, bounce handling, and ISP-specific throttling. An SMTP relay centralizes all delivery management: one our relay instance handles all applications, with consistent authentication, throttling, and monitoring for every message regardless of source application.
Managed SMTP relay infrastructure is the right choice for organizations in several distinct scenarios:
The cost crossover point is typically 500,000–1,000,000 messages per month depending on the transactional service pricing tier. Below this volume, transactional ESPs provide good value with minimal operational overhead. Above this volume, dedicated SMTP relay infrastructure provides both cost advantage and deliverability control that shared services cannot match.
Authentication configuration for SMTP relay is more complex than for direct sending because the relay's IP address must be authorized in the sending domain's SPF record, and the DKIM signing must align with the From: domain for DMARC alignment.
Organizations relaying on behalf of multiple sending domains (agencies, multi-brand operations) require per-domain DKIM key pairs and per-domain DMARC records. our relay handles per-domain DKIM signing natively — each domain has a separate signing key configured in the our relay config. Cloud Server for Email configures per-domain authentication for all relay clients as part of onboarding.
Applications connect to the managed SMTP relay via standard SMTP submission (port 587 with STARTTLS) using SMTP AUTH credentials. The connection is encrypted in transit; the authentication credentials are per-application, enabling per-application rate limiting and monitoring.
Host: [your-relay-server-ip] | Port: 587 | Security: STARTTLS | Auth: PLAIN or LOGIN | Username: [provided during onboarding] | Password: [provided during onboarding]. These parameters work with any application that supports SMTP AUTH — MailWizz, WordPress SMTP plugins, Python smtplib, Node.js nodemailer, PHP mail(), and any other SMTP-capable application.
For development teams already integrated with a commercial transactional email API, switching to dedicated infrastructure typically meant rewriting API calls. We solved this. Our managed SMTP relay exposes an HTTP API layer compatible with Mailgun's, SendGrid's, and Postmark's message submission endpoints — same parameter names, same tag structure, same response format. Your existing code works without modification.
When we say "API-compatible" we mean the following are preserved identically:
The migration path for development teams is:
api.mailgun.net/v3 with our relay endpoint. Same path structure.Authorization: Basic for Mailgun-compatible, Authorization: Bearer for SendGrid-compatible).Libraries and frameworks with native Mailgun or SendGrid support work immediately: Laravel Mail (Mailgun/SendGrid driver), Django Anymail, Node.js mailgun-js / @sendgrid/mail, Ruby mail gem, Go mailgun-go, and any application using the raw HTTP API.
Both are excellent for low-to-medium volume sending. At high volume — above 500K/month — the economics shift: per-message pricing adds up, shared IP pools mean your reputation is affected by co-tenants, and dedicated IPs on their platforms require enterprise contracts. Our relay gives you dedicated IPs, full reputation control, and API compatibility at flat monthly pricing. See our Mailgun comparison and SendGrid comparison for a full breakdown.
If your application processes delivery webhooks — parsing bounce events, complaint notifications, or delivery confirmations — our webhook payloads use the same field names and event types as the major providers. Your existing webhook handler code requires no changes:
We design SMTP relay environments for your specific application stack, volume, and sending domain portfolio. 30-minute technical assessment, no obligation.
A managed SMTP relay must be configured to prevent unauthorized use. An open relay — a server that accepts and forwards email from any source without authentication — is immediately identified by spam monitoring services and blacklisted. Cloud Server for Email's managed relay environments are closed relays by default: SMTP AUTH is required for all injection, and only authenticated connections from authorized clients can inject messages.
Transactional email (password resets, OTP codes, order confirmations) has timing requirements that distinguish it from marketing email. A password reset that takes 10 minutes to deliver is a user experience failure. An OTP code with a 5-minute expiry that arrives in 8 minutes is non-functional.
Cloud Server for Email designs transactional relay configurations for minimum delivery latency: dedicated IPs separate from marketing traffic, generous max-smtp-out settings for major ISPs, and low retry-after values for any deferrals. For organizations with strict transactional SLA requirements (sub-60-second delivery), we configure dedicated virtual MTA pools that route transactional traffic before any marketing queue traffic.
Organizations with multiple applications sending through a central relay benefit from per-application credential design. Each application gets its own SMTP AUTH credentials, enabling per-application monitoring, rate limiting, and attribution in the our relay accounting log.
Application A (e-commerce platform) → SMTP AUTH credentials A → our relay virtual MTA pool A (marketing IPs). Application B (SaaS product) → SMTP AUTH credentials B → our relay virtual MTA pool B (transactional IPs). Application C (CRM) → SMTP AUTH credentials C → our relay virtual MTA pool C (cold outreach IPs). Each application's delivery data is separately attributable in the accounting log and SNDS/Postmaster data.
Standard connection parameters for Cloud Server for Email's managed SMTP relay:
SMTP Host: [assigned at onboarding] | Port: 587 | Encryption: STARTTLS | Authentication: PLAIN or LOGIN | Username: [assigned at onboarding] | Password: [assigned at onboarding] | TLS Version: 1.2 minimum | Compatible with: MailWizz, WordPress (WP Mail SMTP), PHP mail(), Python smtplib, Node.js nodemailer, any RFC 5321 compliant SMTP client.
Organizations evaluating delivery infrastructure often compare SMTP relay, direct MX delivery, and ESP API delivery. The choice depends on application architecture, volume, and operational requirements:
For high-volume applications where IP reputation isolation, GDPR data residency, and full delivery data visibility are requirements, SMTP relay through managed our relay infrastructure is the architecturally correct choice.
A managed SMTP relay requires the same daily monitoring discipline as dedicated bulk email infrastructure. The relay is the delivery layer for all applications — if the relay experiences a deliverability" style="color:#6A47ED;text-decoration:none;border-bottom:1px dashed #6A47ED50">deliverability problem (IP blacklisting, ISP block, authentication failure), every application injecting through it is affected simultaneously.
When a relay IP is blacklisted or blocked at an ISP, Cloud Server for Email's incident response process: (1) Alert to client within 1 hour of detection. (2) Reroute traffic to clean IPs immediately to restore delivery. (3) Investigate root cause in accounting log and application injection data. (4) Submit blacklist removal request with remediation evidence. (5) Post-incident report with root cause and prevention recommendations.
P1 incidents (relay completely down, zero delivery) are responded to 24/7. P2 incidents (significant delivery degradation, ISP block) within 4 hours. See the Service Level Agreement for full incident response commitments.
SMTP relay is included in all Cloud Server for Email managed infrastructure plans. Clients with existing application infrastructure who need only the relay layer (not MailWizz campaign management) can subscribe to a relay-only configuration:
Custom relay configurations (specific IP counts, domain counts, or geographic routing requirements) are available. Contact infrastructure@cloudserverforemail.com.
Organizations evaluating SMTP relay options encounter a wide range of products: shared ESP relay services (SendGrid, Mailgun APIs), managed dedicated relay (Cloud Server for Email), and self-managed open-source relay (Postfix as smarthost). The right choice depends on volume, compliance requirements, and the importance of delivery visibility.
For organizations that need to send more than 500,000 messages per month and for whom inbox placement performance directly affects business outcomes, managed dedicated relay infrastructure provides the IP reputation isolation, per-ISP configuration control, and delivery data visibility that shared relay services cannot. The accounting log data from our relay enables deliverability" style="color:#6A47ED;text-decoration:none;border-bottom:1px dashed #6A47ED50">deliverability optimization that is simply not possible with an API-only delivery service.
The additional operational overhead of managed relay infrastructure compared to a SendGrid API integration is the monitoring discipline: daily accounting log review, Postmaster Tools check, SNDS check. Cloud Server for Email handles this monitoring for all managed clients, reducing the operational burden to reviewing the weekly performance report we provide.
For a detailed comparison of relay options by volume tier and use case, see our PowerMTA vs Mailgun comparison and Postmark vs Dedicated Infrastructure comparison. The Email Infrastructure Glossary defines all technical terms referenced in this page and across the technical reference series.
The Cloud Server for Email technical reference series provides operational depth that complements this service overview. The series is published from production infrastructure management experience — configurations, operational patterns, and delivery optimization approaches validated in live sending environments handling millions of messages daily.
Questions about SMTP relay configuration? The Cloud Server for Email infrastructure team provides technical assessments at no cost. Contact infrastructure@cloudserverforemail.com or +372 602-7190. Operating from the EU (EU) since 2015.
This page is maintained by the Cloud Server for Email infrastructure team and reflects production-validated information current as of 2026. Infrastructure specifications, pricing, and service details are subject to change; contact us for current information specific to your requirements. All infrastructure operates from EU-based dedicated servers in compliance with GDPR Article 28. Service Level Agreement · Infrastructure Specifications · Privacy Policy.
Works with any SMTP-capable application, framework, or platform
Separate keys per sending domain, managed rotation
our relay handles millions of relay messages per day
Per-message accounting log for every relayed message
No per-message fees — unlimited relay volume
GDPR data residency by design, DPA available
Cloud Server for Email operates from the EU (EU) and has managed dedicated email infrastructure since 2015. Every infrastructure client is assigned to a specific infrastructure engineer — not a support ticket queue. Our technical assessment process produces specific configuration recommendations for your sending program, not a generic plan selection. Contact infrastructure@cloudserverforemail.com to begin.
The full technical reference for email infrastructure is available across three interconnected series on this site: the PowerMTA technical reference (53 configuration articles), the MailWizz technical reference (50 configuration articles), and the operational notes series (134 engineering notes on production email infrastructure patterns). Together, these resources represent over a decade of accumulated technical knowledge from managing high-volume email infrastructure in production environments. All content is maintained by the Cloud Server for Email infrastructure team and reflects current production-validated practices.
Organizations evaluating dedicated email infrastructure can also use the Infrastructure Cost Calculator to model the 12-month cost comparison between current ESP pricing and dedicated infrastructure at their specific volume and plan tier. The calculator uses editable pricing fields to match quoted rates, not published list prices.
Infrastructure performance compounds over time. Organizations that establish well-configured, properly monitored dedicated infrastructure in 2026 will have IP and domain reputation histories in 2027 and 2028 that provide measurable competitive advantages in inbox placement. ISPs reward consistent, low-complaint, authenticated sending history with increasingly favorable treatment. The inverse is also true: organizations that operate on shared infrastructure or self-managed infrastructure without daily monitoring accumulate small reputation problems that compound into structural deliverability disadvantages over the same timeline. The infrastructure investment is not just an operational choice — it is a long-term reputation asset that appreciates with consistent compliant operation. Cloud Server for Email's managed infrastructure model ensures that this asset is built and protected through daily operational discipline, not just initial configuration. Every managed client environment is monitored by the same team, using the same daily protocols, for the duration of the engagement. The reputation history built on Cloud Server for Email infrastructure reflects years of consistent professional management — which is visible to ISPs in the same way that years of consistent compliant sending are visible.
