Port 143 (IMAP)
Port 143 is IMAP, the protocol mail apps use to read and sync messages that stay on the server, across phone, laptop and webmail. Connections on 143 start in clear text and upgrade with STARTTLS.
- Service
- IMAP
- Protocol
- TCP
- IANA registry
- Officially assigned: imap (Internet Message Access Protocol), TCP
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 143?
RFC 8314 recommends implicit TLS on port 993 over STARTTLS on 143, because a STARTTLS upgrade can be stripped by an attacker in the middle. Prefer 993 in your mail app and close 143 if no client needs it.
Sources
“Port 143 is already in use”: how to fix it
The error means another program is already listening on port 143. 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 :143taskkill /PID <PID> /FmacOS
lsof -nP -iTCP:143 -sTCP:LISTENkill <PID>Linux
sudo ss -ltnp | grep :143sudo lsof -i :143sudo kill <PID>Replace <PID> with the number from the last column (Windows) or the PID column (macOS, Linux).
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 143 used for?
Port 143 is IMAP, the protocol mail apps use to read and sync messages that stay on the server, across phone, laptop and webmail. Connections on 143 start in clear text and upgrade with STARTTLS.
Is port 143 TCP or UDP?
Port 143 uses TCP. IANA registers it for TCP.
Is it safe to open port 143?
RFC 8314 recommends implicit TLS on port 993 over STARTTLS on 143, because a STARTTLS upgrade can be stripped by an attacker in the middle. Prefer 993 in your mail app and close 143 if no client needs it.
How do I free port 143 if it is already in use?
Find the process: on Windows run netstat -ano | findstr :143 and stop it with taskkill /PID <pid> /F; on macOS or Linux run lsof -i :143 and stop it with kill <pid>.