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Why read this? You don’t need to memorize every RFC to use Relay, but a 5-minute tour of the landscape helps you debug faster — and impress friends at parties.

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Standards & Governance

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IETF

Internet Engineering Task Force – the community that writes the open specifications powering e-mail, HTTP, TLS, etc. All core mail specs are “RFCs” (Request for Comments) published by the IETF.

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RFC

A numbered document that defines a protocol. Example: RFC 5322 specifies e-mail headers. Every server and client that follows the same RFC can inter-operate – that’s why you can fetch Gmail via Apple Mail.

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Anatomy of an email

  • Headers: key-value pairs describing sender, recipients, threading
  • Body: one or more “parts” (plain text, HTML, PDF, images)
email example

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Email Protocols

  • (RFC 3501) — read & organize mail.
  • IDLE (RFC 2177) — real-time notifications.
  • (RFC 1939) — download and delete mail.
  • (RFC 5321) — send mail.
  • (RFC 8620) — modern JSON read/send.

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From Server to App: Who Does What?

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Providers vs. Clients

  • Provider = organization hosting an MTA/MDA (Gmail, Outlook, Proton, your own VPS).
  • Client = anything that logs in to read or send mail (iOS Mail, Superhuman, your script).
Relay treats every provider the same as long as it offers IMAP/SMTP. Your code talks to a single /messages endpoint.

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Email APIs vs. Delivery APIs

Both provide programmable access to email-related operations. Use both together: Resend for blasts, Relay for smart follow-ups.

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Key Terminology in Relay

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Ready?

Head back to Quick-start. We can’t wait to see what you’ll build! Happy hacking — and welcome to programmable e-mail.