E-Mail protocols and data formats

Main protocols

• SMTP
• IMAP
• POP3

Mail structure and content

• MIME
• Mailheaders
• Header encoding (quoted-printable and base64)
• HTML
• uuencode (pre-MIME style)

MIME Content-Transfer-Encoding formats

• Common: 7BIT, 8BIT, BASE64, QUOTED-PRINTABLE
• Old/nonstandard: yencode, xxencode, uuencode

Additional TCP protocols and protocol variations

• Sieve
• JMAP RFC 8620
• LMTP RFC 2033

Security Addons

• TLS
• STARTTLS
• SPF RFC 7208 (+ obsolete SPF DNS record type)
• Sender ID (obsolete SPF alternative) RFC 4406
• DKIM RFC 6376
• DMARC RFC 7489
• Authenticated Received Chain (ARC) RFC 8617
• MTA-STS RFC 8461
• DANE
• REQUIRETLS RFC 8689

Reporting

• Delivery Status Notifications
• Message Disposition Notifications
• Abuse Reporting Format (ARF) RFC 5965
• DMARC reporting (XML)
• SMTP TLS reporting (JSON) RFC 8460

API

• sendmail commandline interface
• mail commandline interface

Login

• SASL RFC 4422
• CRAM-MD5
• OAUTH/XOAUTH
• And many more: https://www.iana.org/assignments/sasl-mechanisms/sasl-mechanisms.xhtml

End-to-End encryption and signing

• OpenPGP RFC 4880 (including related protocols: Keyservers, WKD, ...)
• S/MIME RFC 8551 (including related protocols: X.509, BIMI, ...)

Extra mailheaders

• X-Face, Face
• List-Unsubscribe-Post RFC 8058
• Expires draft-billon-expires

Other

• BURL RFC 4468
• Autoresponder RFC 3834
• autoconfig
• DNS SRV records RFC 6186 RFC 8314
• DNSBL RFC 5782
• Milter