← Back to Operational Notes Archive Engineering Operational Note · Published Production Engineering Memo · Technical Architecture · 14 min read Executive Summary Understanding TLS Requirements and MTA-STS TLS encryption for email in transit became a practical requirement by 2022. MTA-STS extends this by enforcing TLS policy via DNS and HTTPS. Architecture Scope Fleet Scale […]
← Back to Operational Notes Archive Engineering Operational Note · Published Production Engineering Memo · Technical Architecture · 14 min read Executive Summary Soft Bounces That Become Hard Bounces: Time Thresholds and Suppression Logic A 4xx deferral is not a permanent failure — but a 4xx that persists across multiple campaigns eventually is. Architecture Scope […]
← Back to Operational Notes Archive Engineering Operational Note · Published Production Engineering Memo · Technical Architecture · 14 min read Executive Summary The Hidden Impact of Message Size on SMTP Throughput Message size directly affects how many messages a given connection can deliver per unit time. Architecture Scope Fleet Scale High-Volume Infrastructure Production Status […]
← Back to Operational Notes Archive Engineering Operational Note · Published Production Engineering Memo · Technical Architecture · 14 min read Executive Summary Gmail Bulk Sender Requirements: Two Years In — Lessons from Enforcement Production engineering runbook on Gmail Bulk Sender Requirements: Two Years In — Lessons from Enforcement: telemetry signals, MTA queue architecture, and […]
← Back to Operational Notes Archive Engineering Operational Note · Published Production Engineering Memo · Technical Architecture · 14 min read Executive Summary IP Reputation Decay: How Long Before an Idle IP Loses Standing IP reputation is not permanent. An idle IP that stops sending email will lose its accumulated reputation at ISPs over weeks […]
← Back to Operational Notes Archive Engineering Operational Note · Published Production Engineering Memo · Technical Architecture · 14 min read Executive Summary DNS TTL Values and Their Impact on Email Authentication Changes The TTL on authentication DNS records determines how long a change takes to propagate globally. Architecture Scope Fleet Scale High-Volume Infrastructure Production […]
← Back to Operational Notes Archive Engineering Operational Note · Published Production Engineering Memo · Technical Architecture · 14 min read Executive Summary The Anatomy of an SMTP Session: What Happens Between Send and Delivery Every email delivery is an SMTP conversation between two servers. Architecture Scope Fleet Scale High-Volume Infrastructure Production Status 100% Inboxing […]
← Back to Operational Notes Archive Engineering Operational Note · Published Production Engineering Memo · Technical Architecture · 14 min read Executive Summary How Message-ID Structure Affects Deliverability The Message-ID header is more than a unique identifier. ISP filters use its format, domain, and structure as spam signals. Architecture Scope Fleet Scale High-Volume Infrastructure Production […]
← Back to Operational Notes Archive Engineering Operational Note · Published Production Engineering Memo · Technical Architecture · 14 min read Executive Summary Greylisting Behaviour in EU ISPs and How PowerMTA Responds Greylisting mechanics, which EU ISPs use it and how aggressively, PowerMTA configuration for greylist-heavy environments, distinguishing greylisting from… Architecture Scope Fleet Scale High-Volume […]
← Back to Operational Notes Archive Engineering Operational Note · Infrastructure Architecture & Telemetry · 16 min read Executive Engineering Summary Understanding the 7-Layer Delivery Stack: From Application Injection to Inbox The Decoupled Architecture: Production email delivery is not a two-party exchange; it is a strict 7-layer stack spanning application injection, MTA queuing, DNS resolution, […]