Port 587 (SMTP submission)
Port 587 is mail submission (RFC 6409): email apps and websites hand outgoing mail to their provider here, log in, and upgrade the connection with STARTTLS.
- Service
- SMTP submission
- Protocol
- TCP
- IANA registry
- Officially assigned: submission (Message Submission), TCP/UDP
Check if a port is open on your IP
We only check the address you are visiting from, one TCP port at a time. You cannot enter someone else’s address: this is not a port scanner.
Is it safe to open port 587?
Always require authentication and STARTTLS on 587. Providers that block port 25 for customers usually leave 587 open precisely so that legitimate apps can send mail.
Sources
“Port 587 is already in use”: how to fix it
The error means another program is already listening on port 587. Find its process ID (PID), then stop that program or start yours on a different port.
Windows (Command Prompt or PowerShell as administrator)
netstat -ano | findstr :587taskkill /PID <PID> /FmacOS
lsof -nP -iTCP:587 -sTCP:LISTENkill <PID>Linux
sudo ss -ltnp | grep :587sudo lsof -i :587sudo kill <PID>Replace <PID> with the number from the last column (Windows) or the PID column (macOS, Linux).
Port 587 vs 465
Both are for sending mail from an app to your provider. 465 encrypts from the first byte (implicit TLS); 587 starts in clear text and upgrades with STARTTLS. RFC 8314 prefers 465, but 587 is just as widely supported: use whichever your provider lists, and never send without TLS.
Port 465 (SMTPS)Port 587 vs 25
Port 25 is for server-to-server delivery; 587 is for users and apps submitting mail with a login. If your app sends through a provider, use 587 (or 465). Port 25 from home or a cloud VM is often blocked anyway.
Port 25 (SMTP)Why a port can look closed when your server is running
Most home networks sit behind a router doing NAT: the router owns the public address, and your computer has a private one such as 192.168.1.10. A connection from the internet reaches the router and stops there unless you add a port forwarding rule to your computer.
Many mobile and some home providers go further and use carrier-grade NAT (CGNAT): several customers share one public address, and your router itself gets an address from 100.64.0.0/10. Then no rule on your side can open a port; ask the provider for a public IP or use a tunnel or VPN service.
Even with forwarding in place, the firewall on the computer (Windows Defender Firewall, ufw, firewalld) must allow the port, and the program must listen on all interfaces (0.0.0.0), not only on 127.0.0.1.
UDP cannot be checked this way: a UDP service usually stays silent to packets it does not understand, so “no answer” means nothing. This check uses TCP only.
Frequently asked questions
What is port 587 used for?
Port 587 is mail submission (RFC 6409): email apps and websites hand outgoing mail to their provider here, log in, and upgrade the connection with STARTTLS.
Is port 587 TCP or UDP?
Port 587 uses TCP. IANA reserves the number for both TCP and UDP, but in practice only TCP is used.
Is it safe to open port 587?
Always require authentication and STARTTLS on 587. Providers that block port 25 for customers usually leave 587 open precisely so that legitimate apps can send mail.
How do I free port 587 if it is already in use?
Find the process: on Windows run netstat -ano | findstr :587 and stop it with taskkill /PID <pid> /F; on macOS or Linux run lsof -i :587 and stop it with kill <pid>.