Port 25 · SMTP

Port 25 is SMTP, TCP.

Simple Mail Transfer Protocol — server-to-server email delivery (MTA to MTA). Should be blocked outbound for non-mail servers.

Palo Alto

Create the service object for this port, then allow the App-ID on it. Restricting the rule to the port's service object keeps the rule working on a non-default port and stops the App-ID matching on other ports. Replace the placeholders with your own rule name and zones.

set service tcp-25 protocol tcp port 25

set rulebase security rules <rule-name> from <from-zone> to <to-zone> source any destination any application smtp service tcp-25 action allow

Risk and exposure

Rated medium risk. Restrict this port to the sources that need it and keep the service patched; avoid exposing it to the whole internet unless the service is meant to be public.

Check it

• Linux ss -tlnp | grep :25
• PowerShell Test-NetConnection <host> -Port 25
• nmap nmap -p 25 <host>

Related ports

• 465 SMTPS
• 587 SMTP Submission
• 110 POP3
• 143 IMAP
• 993 IMAPS
• 995 POP3S

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