Port 25
smtp · TCP/UDP
| Range | System/well-known port (0–1023) |
|---|---|
| Protocol | TCP/UDP |
| Service | smtp |
| Reference | RFC-ietf-emailcore-rfc5321bis-43 |
| Modified | 2017-06-05 |
Frequently asked questions
What is port 25 used for?
Port 25 is registered with the IANA for smtp (TCP/UDP). Description: Simple Mail Transfer.
Is port 25 a well-known port?
Yes. Port 25 is in the range 0–1023 (system or well-known ports). On Linux and macOS only privileged processes may bind to these ports.
How do I check whether port 25 is open?
Locally on Linux with ss -tulpn | grep ':25\b', on Windows with netstat -ano | findstr :25. To test a remote host use Test-NetConnection <host> -Port 25 in PowerShell or nc -zv <host> 25 on Linux/macOS.
How do I open port 25 in the firewall?
Ubuntu/Debian (ufw): sudo ufw allow 25/tcp. Fedora/RHEL (firewalld): sudo firewall-cmd --add-port=25/tcp --permanent, then firewall-cmd --reload. Windows (PowerShell as administrator): New-NetFirewallRule -DisplayName "Port 25" -Direction Inbound -Protocol TCP -LocalPort 25 -Action Allow.
IANA registry entries
| Protocol | Service | Description (IANA) |
|---|---|---|
| TCP | smtp | Simple Mail Transfer |
| UDP | smtp | Simple Mail Transfer |