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Port 25: SMTP

Common Ports Reference: What runs on port 3389, 445 or 5985: common TCP and UDP ports for web, mail, Windows, AD, databases and VPNs, with security notes.

The ports that come up when opening a firewall, reading a scan or troubleshooting a connection: what runs on each, TCP or UDP, and what to watch out for. Search by number or name. Port numbers and service names were checked against IANA's port number registry; ports that are widely used but assigned by IANA to something else are marked unofficial.

Port number or name

25 is a system port (0-1023): assigned by IANA, and on Linux only root can listen on it. IANA's registry lists everything assigned to it: look up 25 at IANA.

1 of 96 ports

PortProtocolServiceWhat it's forIANA name
25
TCP
SMTP
Mail between servers. Most home and cloud networks block outbound 25 to stop spam; clients should submit on 587 or 465.
smtp
Careful: An SMTP server open to the internet must not relay for strangers (an open relay).

Is port 25 open on your network? Check it → · All common ports →