SMTP

Simple Mail Transfer Protocol moves email between servers and from clients to servers. Related: Mail Server DNS Records Cheat-Sheet, Generate a DKIM Keypair, TLS (Transport Layer Security).

Ports #

PORT	USE
`25`	Server to server delivery (often blocked for clients by ISPs)
`587`	Mail submission with STARTTLS (preferred for clients)
`465`	Submission with implicit TLS
`2525`	Unofficial alternative submission port some providers offer

COMMAND	DESCRIPTION
`nc -zv smtp.example.com 587`	Is the port open?
`dig +short MX example.com`	Mail servers for a domain
`dig +short TXT example.com`	SPF record
`dig +short TXT _dmarc.example.com`	DMARC policy
`swaks --to <TO> --from <FROM> --server smtp.example.com:587 --tls --auth LOGIN --auth-user <USER>`	Send a test mail with `swaks`
`openssl s_client -starttls smtp -connect smtp.example.com:587`	Manual STARTTLS session

bash
echo -ne "mail@example.net" | base64
echo -ne "password" | base64

bash

openssl s_client -starttls smtp -connect smtp.example.com:587

For port 465 use openssl s_client -connect smtp.example.com:465.

smtp

EHLO example.com

smtp

AUTH LOGIN
<your-encoded-username>
<your-encoded-password>

If that’s successful the mail server should return 235 2.7.0 Authentication successful.

smtp

MAIL FROM:<from@example.com>
RCPT TO:<to@example.net>
DATA
Subject: test

Hello
.
QUIT

CODE	MEANING
`220`	Service ready
`235`	Authentication successful
`250`	OK
`354`	Start mail input, end with `.` on its own line
`421`	Service unavailable, try later
`450` / `451`	Temporary mailbox or local error (greylisting)
`535`	Authentication failed
`550`	Mailbox unavailable or rejected (policy, SPF, blocklist)
`554`	Transaction failed