About These Notes

Engineering memos from the Cloud Server for Email infrastructure team. Published monthly since 2015 — documenting operational patterns, configuration decisions, and infrastructure lessons from high-volume email sending environments. 134 notes spanning 2015–2026.

Latest Notes

Engineering Memos — All Years

January 2026
Engineering Memo
Retry Pressure: The Silent Killer of SMTP Reputation at Scale

SMTP · Reputation · Infrastructure

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December 2025
Engineering Memo
Why Inbox Placement Is a Lagging Indicator — And What to Monitor Instead

Monitoring · Deliverability · Metrics

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November 2025
Engineering Memo
Designing ISP-Specific Traffic Isolation for Cold and Bulk Email

Traffic Isolation · ISP · Infrastructure

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October 2025
Engineering Memo
When Manual Intervention Beats Automation in Email Delivery Systems

Operations · Automation · Deliverability

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September 2025
Engineering Memo
The Hidden Cost of Retry Pressure in High-Volume SMTP Systems

SMTP · Retry · Performance

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September 2025
Engineering Memo
SMTP Session Reuse and Pipeline Efficiency in High-Volume Sending

SMTP · Performance · Infrastructure

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August 2025
Engineering Memo
What Breaks First When Volume Increases Too Fast

Volume · Infrastructure · Troubleshooting

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August 2025
Engineering Memo
The Case for Dedicated Bounce Domain Separation

Bounce Processing · Deliverability · DNS

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July 2025
Engineering Memo
Understanding rDNS and EHLO Hostname Alignment in Email Infrastructure

Authentication · DNS · PTR

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July 2025
Engineering Memo
When NOT to Warm an IP

IP Warming · Infrastructure · Operations

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June 2025
Engineering Memo
Why Shared IP Pools Fail at Scale

IP Pools · Scale · Deliverability

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June 2025
Engineering Memo
Monitoring SMTP Connection Latency as a Reputation Signal

Monitoring · Reputation · SMTP

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May 2025
Engineering Memo
Why Your Suppression List Is Your Most Valuable Deliverability Asset

Suppression · List Hygiene · Deliverability

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May 2025
Engineering Memo
Separating Cold, Bulk, and Transactional Traffic at the Infrastructure Level

Traffic Isolation · Infrastructure · Operations

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April 2025
Engineering Memo
Inbox Placement Is a Lagging Indicator

Inbox Placement · Monitoring · Deliverability

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April 2025
Engineering Memo
Soft Bounces That Become Hard Bounces: Time Thresholds for Demotion

Bounce Processing · List Hygiene · SMTP

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March 2025
Engineering Memo
Understanding TLS Requirements and MTA-STS for SMTP Delivery

TLS · Authentication · Security

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February 2025
Engineering Memo
How Message-ID Structure Affects Deliverability at Scale

SMTP · Headers · Deliverability

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January 2025
Engineering Memo
The Anatomy of an SMTP Session: What Every Operator Should Know

SMTP · Infrastructure · Operations

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December 2024
Engineering Memo
DNS TTL Values and Their Impact on Email Authentication Changes

DNS · Authentication · Operations

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November 2024
Engineering Memo
IP Reputation Decay: How Long Before an Idle IP Loses Standing

IP Reputation · Warming · Operations

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October 2024
Engineering Memo
Greylisting Behavior in EU ISPs and How PowerMTA Responds

Greylisting · EU ISPs · PowerMTA

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September 2024
Engineering Memo
Gmail Bulk Sender Requirements: Two Years In — Lessons from Enforcement

Gmail · Authentication · Deliverability

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August 2024
Engineering Memo
The Hidden Impact of Message Size on SMTP Throughput and Reputation

Performance · SMTP · Throughput

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July 2024
Engineering Memo
Send-Time Distribution and Its Effect on ISP Acceptance Rates

Operations · ISP · Throttling

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June 2024
Engineering Memo
Encoding Issues That Break DKIM Signatures in Production

DKIM · Authentication · Troubleshooting

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May 2024
Engineering Memo
Why Dedicated IPs Outperform Shared Pools at Volume

IP Reputation · Infrastructure · Deliverability

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April 2024
Engineering Memo
The Role of List Age in Deliverability: How Stale Lists Harm Reputation

List Hygiene · Reputation · Operations

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March 2024
Engineering Memo
Content Scoring Systems and What Triggers Them in High-Volume Sending

Content · Spam Filtering · Deliverability

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February 2024
Infrastructure Analysis
Capacity Planning for Email Infrastructure: A Methodology

Capacity · Infrastructure · Operations

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January 2024
Infrastructure Analysis
Multi-Datacenter Email Infrastructure and Failover Design

Infrastructure · Redundancy · Operations

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December 2023
Engineering Memo
Why DMARC Reporting Is Underused and What You Miss Without It

DMARC · Authentication · Monitoring

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November 2023
Engineering Memo
The Relationship Between Bounce Rate and ISP Reputation Scoring

Bounce Rate · Reputation · Deliverability

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October 2023
Engineering Memo
Understanding Google Postmaster Tools: IP vs Domain Reputation

Gmail · Reputation · Monitoring

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September 2023
Engineering Memo
Authentication Failure Cascade: When One Missing Record Breaks Everything

Authentication · DMARC · Troubleshooting

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August 2023
Engineering Memo
The Operational Cost of Shared Infrastructure at Scale

Infrastructure · Deliverability · Operations

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July 2023
Engineering Memo
SMTP Error Taxonomy: Building an Internal Bounce Classification System

SMTP · Bounce Processing · Operations

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June 2023
Engineering Memo
High-Frequency Campaign Sending and Its Effect on ISP Relationship

Throttling · ISP · Reputation

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May 2023
Engineering Memo
What ISP Postmaster Teams Actually See When They Review Your Traffic

ISP · Reputation · Operations

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April 2023
Engineering Memo
The Mechanics of Domain Reputation: How Gmail Builds Sender Scores

Gmail · Reputation · Authentication

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March 2023
Engineering Memo
Transactional Email Delivery SLA Design for High-Stakes Sends

Transactional · SLA · Infrastructure

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February 2023
Engineering Memo
Managing Multiple Sender Brands from One Email Infrastructure

Multi-Brand · Infrastructure · DKIM

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January 2023
Engineering Memo
The Economics of IP Warming: Time, Cost, and Opportunity Cost

IP Warming · Operations · Infrastructure

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December 2022
Engineering Memo
Feedback Loop Data as a Segmentation Tool, Not Just a Suppression Tool

FBL · Segmentation · Deliverability

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November 2022
Engineering Memo
Diagnosing Authentication Drift in Long-Running Infrastructures

Authentication · DKIM · Monitoring

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October 2022
Engineering Memo
When Cold Email Infrastructure Damages Your Transactional Sender Score

Cold Email · Transactional · IP Isolation

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September 2022
Engineering Memo
The Infrastructure Changes That Preceded Every Major Deliverability Event

Infrastructure · Deliverability · Operations

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August 2022
Engineering Memo
How to Read Microsoft SNDS Data for Operational Decisions

Microsoft · SNDS · Monitoring

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July 2022
Engineering Memo
The Role of PTR Records in Modern Email Deliverability

PTR · DNS · Authentication

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June 2022
Engineering Memo
Why Email Volume Alone Doesn't Explain Deliverability Outcomes

Deliverability · Reputation · Operations

READ NOTE
May 2022
Engineering Memo
Designing a DKIM Key Management System for Multi-Tenant Infrastructure

DKIM · Multi-Tenant · Security

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April 2022
Engineering Memo
Complaint Rate Spikes: How to Trace Them to Their Source

Complaint Rate · FBL · Deliverability

READ NOTE
March 2022
Engineering Memo
The Long Tail of SMTP Errors: Codes Operators Overlook

SMTP · Error Codes · Troubleshooting

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February 2022
Engineering Memo
Logging Architecture for Email Infrastructure: What to Capture and Why

Logging · Monitoring · Infrastructure

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January 2022
Engineering Memo
Why List Validation at Acquisition Beats Validation at Send Time

List Hygiene · Validation · Deliverability

READ NOTE
December 2021
Engineering Memo
The Compounding Effect of Small Deliverability Improvements

Deliverability · Operations · Reputation

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November 2021
Engineering Memo
Infrastructure Monitoring: What PowerMTA Operators Measure Daily

Monitoring · PowerMTA · Operations

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October 2021
Engineering Memo
ISP-Specific Throttle Behavior and Why One-Size-Fits-All Fails

Throttling · ISP · Configuration

READ NOTE
September 2021
Engineering Memo
Post-Pandemic Email Volume Spikes and Infrastructure Lessons

Volume · Infrastructure · Capacity

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August 2021
Engineering Memo
IPv6 for Email Sending: Readiness Assessment and Operational Risk

IPv6 · Infrastructure · Operations

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July 2021
Engineering Memo
The Myth of Content as the Primary Deliverability Variable

Content · Reputation · Deliverability

READ NOTE
June 2021
Infrastructure Analysis
Infrastructure Isolation for GDPR-Compliant Email Operations in the EU

GDPR · Compliance · EU

READ NOTE
May 2021
Engineering Memo
How Apple Mail Privacy Protection Changed Engagement Metric Reliability

Apple MPP · Engagement · Metrics

READ NOTE
April 2021
Engineering Memo
Why Bounce Handling Is a Shared Responsibility: MTA and Application

Bounce Processing · Architecture · Infrastructure

READ NOTE
March 2021
Engineering Memo
The Operational Difference Between SMTP Relay and MTA Ownership

SMTP Relay · Infrastructure · Operations

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February 2021
Engineering Memo
Building a Deliverability Runbook for Production Incidents

Runbook · Operations · Incident Response

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January 2021
Engineering Memo
Header Hygiene in High-Volume Sending: What ISP Filters Inspect

Headers · ISP Filters · Deliverability

READ NOTE
December 2020
Engineering Memo
The Case for Separate Transactional and Marketing Domains

Transactional · Marketing · Domain Strategy

READ NOTE
November 2020
Engineering Memo
How Retry Storms Occur and Why They Are Hard to Detect Early

Retry · Queue · Operations

READ NOTE
October 2020
Engineering Memo
Measuring Email Infrastructure ROI Beyond Delivery Rate

ROI · Infrastructure · Business

READ NOTE
September 2020
Engineering Memo
Infrastructure Response to Sudden Volume Spikes: A Technical Review

Volume Spikes · Capacity · Infrastructure

READ NOTE
August 2020
Engineering Memo
The Deliverability Impact of Sending from New Geographic Regions

Geolocation · IP Reputation · Infrastructure

READ NOTE
July 2020
Engineering Memo
Configuring Email Infrastructure for High-Frequency Transactional Sends

Transactional · Performance · Configuration

READ NOTE
June 2020
Engineering Memo
SMTP Rate Limiting Strategies for Protecting ISP Relationships

Rate Limiting · ISP · Throttling

READ NOTE
May 2020
Engineering Memo
DMARC Enforcement Migration: Lessons from Large-Scale Deployments

DMARC · Authentication · Migration

READ NOTE
April 2020
Engineering Memo
The Queue as a Diagnostic Tool: Reading Queue Depth Patterns

Queue · Monitoring · Troubleshooting

READ NOTE
March 2020
Engineering Memo
Email Infrastructure for E-Commerce: Seasonal Volume Management

E-Commerce · Seasonal · Capacity

READ NOTE
February 2020
Engineering Memo
How to Run a Deliverability Audit Without a Third-Party Tool

Audit · Deliverability · Operations

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January 2020
Engineering Memo
Separating Signal from Noise in SMTP Accounting Logs

Logging · Analysis · Operations

READ NOTE
December 2019
Infrastructure Analysis
The Compliance Infrastructure Behind GDPR and CAN-SPAM Alignment

Compliance · GDPR · CAN-SPAM

READ NOTE
November 2019
Engineering Memo
Why Dedicated Infrastructure Scales Differently Than Shared ESPs

Dedicated Infrastructure · ESP · Deliverability

READ NOTE
October 2019
Engineering Memo
Operational Protocols for Managing IP Reputation Events

IP Reputation · Incident Response · Operations

READ NOTE
September 2019
Engineering Memo
List Hygiene Practices That Preempt Deliverability Problems

List Hygiene · Deliverability · Prevention

READ NOTE
August 2019
Engineering Memo
The Infrastructure Behind Reliable One-Time-Password Delivery

OTP · Transactional · Reliability

READ NOTE
July 2019
Engineering Memo
How Engagement Data Should Drive Sending Infrastructure Decisions

Engagement · Segmentation · Deliverability

READ NOTE
June 2019
Engineering Memo
SMTP Authentication Mechanisms: PLAIN, LOGIN, and CRAM-MD5 Compared

SMTP · Authentication · Security

READ NOTE
May 2019
Engineering Memo
Why Reputation Recovery Takes Longer Than Reputation Damage

Reputation · Recovery · Operations

READ NOTE
April 2019
Engineering Memo
The Technical Requirements for High Inbox Placement at Gmail

Gmail · Inbox Placement · Authentication

READ NOTE
March 2019
Engineering Memo
Building a Sending Pool Strategy for Mixed Traffic Types

IP Pools · Traffic Isolation · Infrastructure

READ NOTE
February 2019
Engineering Memo
How Blacklist Operators Identify and List SMTP Sending Infrastructure

Blacklists · DNSBL · Reputation

READ NOTE
January 2019
Engineering Memo
The Operational Risk of Single-IP Sending at Scale

IP Risk · Redundancy · Infrastructure

READ NOTE
December 2018
Engineering Memo
What Changes When You Move from Shared to Dedicated Infrastructure

Dedicated Infrastructure · Migration · Operations

READ NOTE
November 2018
Engineering Memo
Testing Methodologies for Email Infrastructure Before Production

Testing · QA · Infrastructure

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October 2018
Engineering Memo
Why All SMTP Errors Are Not Equal: A Classification Framework

SMTP · Error Classification · Operations

READ NOTE
September 2018
Engineering Memo
Designing for Zero Downtime in Email Sending Infrastructure

High Availability · Infrastructure · Design

READ NOTE
August 2018
Engineering Memo
The Role of Dedicated Infrastructure in High-Stakes Transactional Email

Transactional · Dedicated · Reliability

READ NOTE
July 2018
Engineering Memo
How ISP Filtering Systems Evolved: From Content to Reputation

ISP · History · Reputation

READ NOTE
June 2018
Engineering Memo
The Case for Real-Time Bounce Processing at the MTA Layer

Bounce Processing · MTA · Real-Time

READ NOTE
May 2018
Engineering Memo
Understanding SPF Alignment Failure Modes and How to Prevent Them

SPF · DMARC · Authentication

READ NOTE
April 2018
Engineering Memo
IP Pool Architecture for High-Volume B2C Email Operations

IP Pools · B2C · Architecture

READ NOTE
March 2018
Engineering Memo
Why Delivery Rate Is Not the Same as Inbox Placement Rate

Delivery Rate · Inbox Placement · Metrics

READ NOTE
February 2018
Engineering Memo
What Operators Get Wrong About SMTP Connection Reuse

SMTP · Performance · Configuration

READ NOTE
January 2018
Engineering Memo
Operational Monitoring for Email Infrastructure: A Starting Framework

Monitoring · Operations · Framework

READ NOTE
December 2017
Engineering Memo
The Economics of Email Infrastructure: Build vs. Buy Analysis

Economics · Infrastructure · Business

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November 2017
Engineering Memo
SMTP Timeout Configuration and Its Effect on Delivery Behavior

SMTP · Configuration · Timeouts

READ NOTE
October 2017
Engineering Memo
Why Authentication Is Table Stakes, Not a Differentiator

Authentication · DKIM · SPF

READ NOTE
September 2017
Engineering Memo
The Infrastructure Foundations of Cold Email at Scale

Cold Email · Infrastructure · Scale

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August 2017
Engineering Memo
Building Observable Email Infrastructure: Logging and Alerting Design

Observability · Logging · Alerting

READ NOTE
July 2017
Engineering Memo
DKIM: From Theory to Production — An Operator's Guide

DKIM · Authentication · Operations

READ NOTE
June 2017
Engineering Memo
The Relationship Between List Quality and Infrastructure Performance

List Quality · Deliverability · Infrastructure

READ NOTE
May 2017
Engineering Memo
How to Evaluate Email Infrastructure Providers: A Technical Checklist

Vendor Selection · Infrastructure · Checklist

READ NOTE
April 2017
Engineering Memo
Understanding SMTP Pipelining and Its Deliverability Implications

SMTP · Pipelining · Performance

READ NOTE
March 2017
Engineering Memo
Why the Sending Domain Matters as Much as the Sending IP

Domain Reputation · IP Reputation · Authentication

READ NOTE
February 2017
Engineering Memo
Feedback Loops: How They Work and Why Every Operator Needs Them

FBL · Reputation · Operations

READ NOTE
January 2017
Engineering Memo
The Technical Anatomy of Inbox Placement

Inbox Placement · Authentication · Reputation

READ NOTE
December 2016
Engineering Memo
What High-Volume Operators Know About ISP Relationships

ISP Relationships · Operations · Reputation

READ NOTE
November 2016
Infrastructure Analysis
Designing Email Infrastructure for Regulatory Compliance in Europe

GDPR · Compliance · EU

READ NOTE
October 2016
Engineering Memo
The Cost of Poor Bounce Handling: A Technical and Reputational Analysis

Bounce Handling · Deliverability · Operations

READ NOTE
September 2016
Engineering Memo
Why Email Deliverability Requires Infrastructure Expertise, Not Just Tools

Deliverability · Infrastructure · Expertise

READ NOTE
August 2016
Engineering Memo
The Operational Case for PowerMTA vs Postfix at Production Volume

PowerMTA · MTA · Infrastructure

READ NOTE
July 2016
Engineering Memo
SMTP Header Analysis: What Receiving Servers Evaluate on Every Message

Headers · SMTP · ISP Filters

READ NOTE
June 2016
Engineering Memo
Building a Multi-Brand Email Sending Architecture

Multi-Brand · Architecture · DKIM

READ NOTE
May 2016
Engineering Memo
How Volume Thresholds Trigger ISP Filtering Behavior Changes

Volume · ISP · Throttling

READ NOTE
April 2016
Engineering Memo
The Relationship Between SPF Record Complexity and Delivery Failure Risk

SPF · DNS · Configuration

READ NOTE
March 2016
Infrastructure Analysis
What We Learned Building Email Infrastructure for European Senders

EU · Infrastructure · Lessons

READ NOTE
February 2016
Engineering Memo
Sender Identification: How ISPs Attribute Mail Streams to Organizations

ISP · Sender Identity · Reputation

READ NOTE
January 2016
Engineering Memo
Deliverability Consulting vs Infrastructure Management: The Difference

Infrastructure · Consulting · Deliverability

READ NOTE
December 2015
Engineering Memo
Why Email Infrastructure Decisions Compound Over Time

Infrastructure · Long-Term · Strategy

READ NOTE
November 2015
Engineering Memo
The Technical Requirements for Large-Scale Newsletter Sending

Newsletter · Scale · Infrastructure

READ NOTE
October 2015
Engineering Memo
Logging as a First-Class Concern in Email Infrastructure Design

Logging · Design · Infrastructure

READ NOTE
September 2015
Infrastructure Analysis
Why We Built Our Infrastructure on PowerMTA

PowerMTA · Infrastructure · History

READ NOTE
August 2015
Infrastructure Analysis
The Founding Principles of Cloud Server for Email

Company · Infrastructure · Philosophy

READ NOTE
July 2015
Infrastructure Analysis
Email Infrastructure for High-Volume Sending: A Primer

Primer · Infrastructure · Introduction

READ NOTE
June 2015
Engineering Memo
Understanding the Email Delivery Stack: From MTA to Inbox

MTA · Delivery Stack · Infrastructure

READ NOTE

Archive by Year

Technical References

Technical References

Infrastructure Questions?

These notes reflect what we work on daily. If any of this resonates with your operational situation, reach out for a technical consultation.