Email Protocols

This post presents some aspects related to electronic mail (email or e-mail) protocols.

Email Operational Protocols

  • Inbound e-mails
    • POP3
    • IMAP
  • Outbound e-mails
    • SMTP


An open relay is an insecure SMTP server that does not authenticate sneders before accepting and relaying email and are typically quickly exploited to send email for spammers.

Email Security Protocols

These security protocols are used for example with SMTP:

  • E-mail encryption
    • S/MIME
    • PGP
  • Inbound e-mail authentication
    • DKIM
    • SPF
    • DMARC
  • Secure Outbound e-mail communication
    • Implicit STMPS

E-mail Encryption

E-mail Encryption Protocols featured on this post:

  • S/MIME
  • PGP


Secure Multipurpose Internet Mail Extension (S/MIME) is an email security standard.

It provides:

  • Confidentiality: Public Key Cryptography Standard (PKCS) encryption
  • Authentication: X.509 digital certificates issued by trusted third-party certificate authorization (CAs).

S/MIME types of messages:

  • Signed messages: provides integrity, sender authentication, and non-repudiation.
  • Secure enveloped method: provides recipient authentication and confidentiality.

Secure enveloped method is specific to S/MIME.

S/MIME secure email format uses the P7S for encrypted email messages. If the recipient does not have a mail reader that supports S/MIME, the message will appear with an attachement named smime.p7s.

The main difference between S/MIME and PGP is that S/MIME requires a PKI while PGP does not.


Pretty Good Privacy (PGP) is a peer-to-peer public-private key-based email system.

When implemented, the authenticity of the users’ public keys are attested to by others in a circle of trust, the role of a Certificate authority is not needed.

The main difference between PGP and S/MIME that S/MIME requires a PKI while PGP does not.

Privacy Enhanced Email


Privacy-Enhanced Mail (PEM) is a file format for storing and sending cryptographic keys, certificates, and other data.

It was based on a set of 1993 IETF standards defining “privacy-enhanced mail”.

The PEM format was formalized by the IETF in RFC 7468, though it started as a de facto file format.

While the original standards were never broadly adopted and were supplanted by PGP and S/MIME, the textual encoding they defined became very popular.



MIME Object Security Services (MOSS) / Privacy-Enhanced Mail MIME (PEM-MIME) is a protocol that uses the multipart/signed and multipart/encrypted framework to apply digital signature and encryption services to MIME objects.

It was never largely deployed and the project is abandoned mainly due to the popularity of PGP.

Inbound E-mail Authentication

Inbound e-mail authentication protocols featured on this post:

  • DKIM
  • SPF


Domain Keys Identified Mail (DKIM) is a mean to assert that valid mail is sent by an organization through verification of domain name identity.

It is used to detect spam and email spoofing.


Sender Policy Framework (SPF) is also a mean that valid is sent by an organization through its SMTP server.

The solution checks that inbound messages originate from a host authorized to send messages by the owner of the SMTP origin domain.

Example: the systems receives an e-mail from SPF would check with the admin of that user jane.doe is authorized to send messages.


Domain Message Authentication Reporting and Conformance (DMARC) is a DNS-based email authentication system. It is built on top of DKIM and SPF.

E-mail servers can verify if a received message is valid by following the DNS-based instructions.

It is intended to protect against business email compromise (BEC), phishing, and other email scams.

Secure Outbound E-mail Communication

Secure outbound e-mail communication protocols featured on this post:

  • Implicit SMTPS

Implicit SMTPS


SMTPS is the TLS-encrypted form of SMTP. Implicit SMTPS involves establishing a secure connection from the outset.

Implicit SMTPS encrypts the entire SMTP session, including the handshake process, using SSL/TLS.

Implicit SMTPS assumes that the connection will be encrypted from the beginning.

It uses port 465.

It is not much used nowadays, compared to STARTTLS, and is consided outdated.


STARTTLS, explicit TLS or opportunistic TLS for SMTP is an SMTP command that negotiates encryption after the initial connection is made.

It uses port 587.

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External References

  • M. Chapman et al; “CISSP Study Guide 9th Edition”, section “Email Security Solutions”, pp. 599-601; Wiley, 2021
  • Inbound e-mail authentication

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