Protocols along the path

SMTP submits and transfers mail. RFC 5322 describes the basic message format. MIME adds content types, international text encodings, multipart bodies, and attachments.

POP3 downloads messages from a maildrop. IMAP synchronizes server-side mailboxes. LMTP is an SMTP-like final-delivery protocol, and Sieve is a server-side filtering language.

JMAP exposes synchronized mail data through HTTP and JSON. These technologies overlap at the edges, but each was designed for a different responsibility.

Put them on one path:

compose RFC 5322 + MIME message
  -> submit with SMTP
  -> relay with SMTP
  -> deliver with SMTP or LMTP
  -> filter with Sieve
  -> store in a mailbox
  -> access with POP3, IMAP, or JMAP

RFC 5322 and MIME are formats, not delivery sessions. IMAP and JMAP manipulate stored mail, but neither replaces the Internet SMTP relay between unrelated domains. Sieve runs near delivery; it does not need a desktop client online.

This distinction matters during failure analysis. A malformed MIME boundary can make an attachment unreadable even though SMTP delivery succeeded. An IMAP synchronization bug can hide a delivered message without changing its SMTP trace.

Classify these failures by layer: 550 during RCPT TO, an attachment that decodes incorrectly, and a message visible in webmail but missing on a phone.