25/TCP, 465/TCP, 587/TCP (SMTP/S)

SMTP, Simple Mail Transfer Protocol. SMTPS, Simple Mail Transfer Protocol Secure.

Open relay

nmap -p 25,465,587 --script=smtp-open-relay.nse <target>

telnet <target> 25
HELO idontexist
MAIL FROM: [email protected]
RCPT TO: [email protected]
DATA
Subject: Test open relay
Test message
.
QUIT