Port 25 — SMTP

TCP well-known

What runs on port 25

Server-to-server mail transfer.

Security considerations

Most residential ISPs and cloud providers block outbound 25 to limit spam. For sending mail from an application use port 587 with authentication.

Checking whether something is listening

# Linux / macOS — what is bound to the port
sudo lsof -i :25
sudo ss -lntp | grep :25

# Windows
netstat -ano | findstr :25
Get-NetTCPConnection -LocalPort 25

# is it reachable from outside?
nc -zv example.com 25
curl -v telnet://example.com:25

Freeing the port

# find the process, then stop it
sudo lsof -ti :25 | xargs kill        # Linux / macOS
netstat -ano | findstr :25            # note the PID, then:
taskkill /PID <pid> /F                    # Windows

Should this port be open to the internet?

Only on a mail server. If this port is open on something that is not intentionally handling mail, you may be running an open relay — which will get the IP blacklisted quickly.

Quick reference

Frequently asked questions

Other common ports

• Port 20 — FTP data
• Port 21 — FTP control
• Port 22 — SSH / SFTP
• Port 23 — Telnet
• Port 53 — DNS
• Port 67 — DHCP server
• Port 68 — DHCP client
• Port 69 — TFTP
• Port 80 — HTTP
• Port 110 — POP3
• Port 123 — NTP
• Port 143 — IMAP
• Port 161 — SNMP
• Port 389 — LDAP
• Port 443 — HTTPS
• Port 445 — SMB
• Port 465 — SMTPS
• Port 514 — Syslog

All port numbers