Port 25 SMTP
Port 25 is the IANA-assigned smtp port for Simple Mail Transfer (TCP / UDP).
What is port 25 used for?
Port 25 is assigned to smtp and uses TCP / UDP. The IANA registry describes the service as “Simple Mail Transfer”. It sits in the well-known range, which operating systems reserve for core network services, so software expects it at this number by default.
Ports are the transport-layer address a service listens on. A client that wants to reach smtp connects to port 25. A TCP connection is refused when nothing is listening, while a UDP request simply goes unanswered. That is why a port conflict appears whenever two processes try to bind the same number.
Use the commands below to confirm whether port 25 is actually in use before you change firewall rules. Blocking the port stops new inbound traffic at the firewall, but it does not stop a local process from listening, and it does not remove the service from the machine.
Source: IANA Service Name and Transport Protocol Port Number Registry. Registered assignee: [IESG]. Reference: [RFC-ietf-emailcore-rfc5321bis-43].
Check and block port 25
Copy-ready commands for the protocols assigned to this port. Run the check first, then block only what should not be reachable.
Check on Linux
sudo ss -tlpn 'sport = :25'
sudo ss -ulpn 'sport = :25'Check on Windows
Get-NetTCPConnection -LocalPort 25 -State Listen -ErrorAction SilentlyContinue
Get-NetUDPEndpoint -LocalPort 25 -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 25/tcp
sudo ufw deny 25/udpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 25 -j DROP
sudo iptables -A INPUT -p udp --dport 25 -j DROPThese commands are examples for a typical host. Check the listener before you change firewall rules, and persist iptables rules with iptables-save or netfilter-persistent. On Windows, use Windows Defender Firewall or New-NetFirewallRule for persistent blocks. The iptables rules here cover IPv4; mirror them with ip6tables if IPv6 is enabled.
Is port 25 safe to expose?
An exposed SMTP service can be abused for spam relay or address harvesting if authentication and relay rules are weak. Expose this port only when the service is actually needed. Keep it patched, prefer an encrypted alternative, and limit source addresses with a firewall allowlist.
Where port 25 is usually used
Most traffic on port 25 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Mail clients sending messages through a relay or fetching them from a mailbox.
- Mail servers exchanging messages for a domain or syncing folders with clients.
- Deliverability, authentication and TLS tests that connect directly to the mail service.
Probe the SMTP listener
A TCP probe confirms the relay is reachable; the SMTP banner and TLS policy appear only after you connect.
nc -vz mail.example.com 25Common problems with port 25
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 25. A timeout more often means a firewall drop, a wrong host or a service bound to a different interface.
Port already in use
Find the process holding the port with sudo ss -tlpn 'sport = :25' or sudo lsof -i :25. Stop that process or configure it to bind another port; killing it without identifying it can take down a production service.
Firewall rule did not survive a reboot
UFW and Windows Firewall keep rules across restarts. Raw iptables rules do not: save them with iptables-save or the netfilter-persistent package, and re-apply after upgrades that reset the ruleset.
The port is blocked but still reachable
A firewall rule filters inbound traffic; it does not close the listener. Confirm with the check command above and bind the service to 127.0.0.1 when only local clients need it.
Port 25 at a glance
| Port number | 25 |
|---|---|
| Service name | smtp |
| Transport protocol | TCP / UDP |
| Category | |
| Range | Well-known port (0–1023) |
| Risk level | Medium |
| Registered assignee | [IESG] |
| Reference | [RFC-ietf-emailcore-rfc5321bis-43] |
| Registry description | Simple Mail Transfer |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 25 used for?
Port 25 (SMTP) uses TCP / UDP. Simple Mail Transfer.
How do I check if port 25 is open?
On Linux run `sudo ss -tlpn 'sport = :25'` and `sudo ss -ulpn 'sport = :25'`. On Windows run `Get-NetTCPConnection -LocalPort 25 -State Listen -ErrorAction SilentlyContinue` and `Get-NetUDPEndpoint -LocalPort 25 -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 25; empty output usually means nothing is bound to it.
How do I block port 25?
With UFW run `sudo ufw deny 25/tcp` and `sudo ufw deny 25/udp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 25 -j DROP` and `sudo iptables -A INPUT -p udp --dport 25 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 25 safe to expose?
An exposed SMTP service can be abused for spam relay or address harvesting if authentication and relay rules are weak. Nevertheless, expose this port only when the service is actually needed. Keep it patched, prefer an encrypted alternative, and limit source addresses with a firewall allowlist.
Related ports
Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy