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SMTP
Service

SMTP

Mail transfer service for sending emails.

:25 :465 :587 5 guides 3 phases
1 min read Updated: Jul 13, 2026

What is SMTP?

SMTP (Simple Mail Transfer Protocol) is used to send email from clients to mail servers, and between mail servers.

Common security issues

  • Open relay misconfiguration (spammers love this).

  • Username enumeration (VRFY/EXPN).

  • Weak TLS enforcement.

  • Missing SPF/DKIM/DMARC leading to spoofing.

  • Brute-force authentication.

Default ports

  • 25 – mail relay

  • 465 – SMTPS (implicit TLS)

  • 587 – client submission

Enumeration

Enumerate SMTP banners, auth methods, and NTLM info via nmap and openssl

1

Vulnerability Discovery

Guides on testing SMTP for open relays and user enumeration

2

Exploitation

Guides on relay abuse, spoofed email delivery, and auth brute force

2