Protocols

Clients talk to Mail Transfer Agents (MTAs / Servers) in a client-server relationship, using IMAP or POP(3)

MTAs talk to each other in a peer-to-peer relationship, using SMTP (or more likely, ESMTP (extended))

SMTP

Defined in RFC 5321

This is the “envelope” that smtp servers use to send mail to each other.

IMF / Internet Message Format

Defined in RFC 5322

This is the format of the mail itself, which is transported over smtp.