Simple Mail Transfer Protocol

Simple Mail Transfer Protocol (SMTP) is an Internet standard for electronic mail (e-mail) transmission across Internet Protocol (IP) networks. SMTP was first defined by RFC 821 (1982), and last updated by RFC 5321 (2008) which includes the extended SMTP (ESMTP) additions, and is the protocol in widespread use today. SMTP uses TCP port 25.

While electronic mail servers and other mail transfer agents use SMTP to send and receive mail messages, user-level client mail applications typically only use SMTP for sending messages to a mail server for relaying. For receiving messages, client applications usually use either the Post Office Protocol (POP) or the Internet Message Access Protocol (IMAP) or a proprietary system to access their mail box accounts on a mail server.

• Simple Mail Transfer Protocol [EN] @ Wikipedia
• Simple Mail Transfer Protocol [DE] @ Wikipedia

Documentation

Request for Comments (RFC)

• 3207 - SMTP Service Extension for Secure SMTP over Transport Layer Security [EN]
• 4954 - SMTP Service Extension for Authentication [EN]
• 5248 - A Registry for SMTP Enhanced Mail System Status Codes [EN]
• 5321 - Simple Mail Transfer Protocol [EN]
• 6409 - Message Submission for Mail [EN]
• 6531 - SMTP Extension for Internationalized Email [EN]
• 8461 - SMTP MTA Strict Transport Security (MTA-STS) [EN]

Implementations

Server

2 pages found:

• OpenSMTPD
• Postfix

Client

1 pages found:

• Mail

Examples

Send a email directly to a SMTP server using telnet

telnet mail.raysoft.loc 25
220 mail.raysoft.loc ESMTP

HELO oxygen.raysoft.loc
250 mail.raysoft.loc Hello oxygen.raysoft.loc [10.0.0.101]

MAIL FROM: <alex@raysoft.loc>
250 <alex@raysoft.loc> is syntactically correct

RCPT TO: <melman@raysoft.loc>
250 <melman@raysoft.loc> verified

DATA
354 Enter message, ending with "." on a line by itself
From: Alex the Lion <alex@raysoft.loc>
To: Melman the Giraffe <melman@raysoft.loc>
Subject: Who are you?

Hi Melman,
Who are you?

C U,
Alex
.
250 OK

QUIT
221 mail.raysoft.loc closing connection

Send a email directly to a SMTP server with authentication using telnet

echo -en '\0alex@raysoft.loc\0********' | openssl enc -base64

Output:

AGFsZXhAcmF5c29mdC5sb2MAKioqKioqKio=

telnet mail.raysoft.loc 25
220 mail.raysoft.loc ESMTP

EHLO oxygen.raysoft.loc
250 mail.raysoft.loc Hello oxygen.raysoft.loc [10.0.0.101]
250-SIZE 52428800
250-PIPELINING
250-AUTH PLAIN LOGIN
250-STARTTLS
250 HELP

AUTH PLAIN AGFsZXhAcmF5c29mdC5sb2MAKioqKioqKio=
235 Authentication succeeded

MAIL FROM: <alex@raysoft.loc>
250 <alex@raysoft.loc> is syntactically correct

RCPT TO: <melman@raysoft.loc>
250 <melman@raysoft.loc> verified

DATA
354 Enter message, ending with "." on a line by itself
From: Alex the Lion <alex@raysoft.loc>
To: Melman the Giraffe <melman@raysoft.loc>
Subject: Who are you?

Hi Melman,
Who are you?

C U,
Alex
.
250 OK

QUIT
221 mail.raysoft.loc closing connection