| Protocol | Port | Description |
|---|---|---|
| SMTP | 25 | |
| SSL/TLS | 465 | |
| StartTLS | 587 |
MUA → MSA → MTA → internet → MTA → MDA → MUA
{% code overflow="wrap" %}
Outlook → Exchange → firewall → internet → SMTP-Server of the receiving side → mail-server
{% endcode %}
SMTP Commands
| Command | Description | RFC |
|---|---|---|
| HELO | Identifies the sending SMTP server | RFC 5321 |
| EHLO | Extended HELO, provides more information | RFC 5321 |
| MAIL FROM | Specifies the sender’s email address | RFC 5321 |
| RCPT TO | Specifies the recipient’s email address | RFC 5321 |
| DATA | Initiates the message data transmission | RFC 5321 |
| RSET | Resets the session to initial state | RFC 5321 |
| VRFY | Requests verification of an email address | RFC 5321 |
| EXPN | Requests the expansion of a mailing list | RFC 5321 |
| NOOP | No operation, used for testing or keep-alive | RFC 5321 |
| QUIT | Closes the SMTP session | RFC 5321 |
| AUTH | Initiates authentication process | RFC 4954 |
| STARTTLS | Initiates a secure TLS session | RFC 3207 |
| HELP | Requests help information from the server | RFC 5321 |
RFC 821 – Simple Mail Transfer Protocol
Protocol definition for SMTP. This document covers the model, operating procedure, and protocol details for SMTP.
RFC 1869 – SMTP Service Extensions
Definition of the ESMTP extensions for SMTP. This describes a framework for extending SMTP with new commands, supporting dynamic discovery of the commands provided by the server, and defines a few additional commands.