Port25— SMTP relay
TCP 25 carries SMTP mail transfer, commonly between mail servers. Mail clients should use the submission endpoint specified by their provider.
Registry assignments, software defaults and local listeners are different things. A port number alone does not identify or secure a service.
Configuration and troubleshooting
Separate inbound delivery, outbound relay and client submission. Inspect mail logs and provider egress rules before assuming a local port conflict.
Important limitations
Do not configure an open relay. A reachable socket says nothing about recipient policy or successful mail delivery.
Read-only checks: run on the server host
Choose the commands for your OS. Check TCP and UDP separately. No result is not a lasting availability guarantee; inspect host and container separately.
Linux
sudo ss -ltnp 'sport = :25'macOS
sudo lsof -nP -iTCP:25 -sTCP:LISTENWindows PowerShell
Get-NetTCPConnection -LocalPort 25 -State ListenSources and review
Sources checked on:
This review covers the explanations and sources on this page, not your device or every community software entry below.
Software Using This Port (4 software)
Community software entries: links are references, not individual verification.
SMTP Mail
Simple Mail Transfer Protocol
Usage Suggestions
- Before using port 25, please confirm it is not occupied by other services
- If you encounter port conflicts, consider using other ports or stopping the occupying service
Frequently asked questions
What is port 25 used for?
Port 25 sits in the Email & Messaging category. This page lists the registered purpose and the software commonly associated with it; to see what is actually listening on a specific machine, check the process with ss/netstat.
Is port 25 TCP or UDP?
The registered protocol on this page is TCP. The same number behaves independently on each protocol: TCP may be listening while UDP is idle, so check them separately.
How do I check if port 25 is in use?
On Linux: sudo ss -tlnp 'sport = :25' for an exact port match. On macOS: sudo lsof -nP -iTCP:25 -sTCP:LISTEN. On Windows PowerShell: Get-NetTCPConnection -LocalPort 25 -State Listen. A listener counts as confirmed only when you can see the bind address and process name; without root/admin you will not see other users' processes.
Is port 25 secure? Should I open it in my firewall?
A port number has no inherent security — it depends on whether the listening service is correctly configured and whether it needs exposure at all. Keep loopback/internal-only services firewalled; where public access is required, pair it with source restrictions, VPN or authentication. The caution section on this page lists port-specific risks.
Quick Info
- Port
- 25
- Protocol
- TCP
- Categories
- Email & Messaging
- Usage Status
- 4 software in use
Related Ports
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