Email engineering guides
Guides to the parts of email that actually cause problems — authentication failures, spam placement, duplicate sends, and giving AI agents sending access without regretting it. Every piece answers its title question in the first paragraph.
Troubleshooting
Specific failures, with the causes ordered by how often they're it.
- How to fix an SPF record that isn't foundWhy an SPF lookup returns nothing even after you've added the record, and how to diagnose it: propagation, wrong host, quoting, and the one-SPF-record rule.
- Why DKIM verification fails (and how to fix each cause)The concrete reasons a DKIM check fails after you've published the key: selector mismatch, a truncated public key, DNS flattening, and message modification in transit.
- Why your transactional email goes to spamA diagnostic order of operations for transactional mail landing in spam: authentication, reputation, content, and the engagement signals you can't see.
Concept
The distinctions that change how you build.
- Transactional vs marketing email: what's the difference?The practical distinction between transactional and marketing email — consent, unsubscribe obligations, reputation risk — and when one tool for both is the right call.
- What is an idempotency key, and when do you need one?How idempotency keys prevent duplicate sends when a request is retried, how to choose one, and the failure modes they don't cover.
Architecture
How email infrastructure should behave, and why.
AI agents
Email for AI agents — scopes, gates, and what can go wrong.
- How to let an AI agent send email safelyThe concrete controls that make agent-initiated email safe: scoped credentials, a verified-domain gate, idempotency, approval before send, and an audit trail.
- What is an email MCP server?How Model Context Protocol exposes email sending as agent tools, what a well-designed email MCP surface looks like, and how authentication and scopes work.
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