Port 587 — SMTP submission

TCP well-known

What runs on port 587

The port applications and mail clients use to send outbound mail, with authentication and STARTTLS.

Security considerations

This is the correct port for application email. Port 25 is for server-to-server relay and is widely blocked.

Checking whether something is listening

# Linux / macOS — what is bound to the port
sudo lsof -i :587
sudo ss -lntp | grep :587

# Windows
netstat -ano | findstr :587
Get-NetTCPConnection -LocalPort 587

# is it reachable from outside?
nc -zv example.com 587
curl -v telnet://example.com:587

Freeing the port

# find the process, then stop it
sudo lsof -ti :587 | xargs kill        # Linux / macOS
netstat -ano | findstr :587            # 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 25 — SMTP
• 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

All port numbers