25
Email transmission between mail servers
Simple Mail Transfer Protocol
What is port 25?
SMTP on port 25 is the foundation of email on the internet. It is used exclusively for server-to-server communication (MTA to MTA). ISPs almost always block outbound port 25 from residential connections to prevent spam. For sending email from an application or client, use port 587 (SMTP Submission) or 465 (SMTPS). Checking whether a mail server has port 25 open is commonly done when debugging DNS MX records.
Quick checks
$
nc -zv example.com 25
$
nmap -p 25 example.com
$
telnet example.com 25