What is port 143 used for?
Internet Message Access Protocol: email clients read and sync mail on the server.
Should port 143 be open?
Open only if you need it. Expose it only when required, with encryption and strong authentication, ideally limited to known IP addresses. Without STARTTLS, IMAP on 143 sends passwords in plain text. Prefer IMAPS on port 993.
See what is listening on port 143
# Linux
sudo ss -tlnp | grep ':143 '
# macOS
sudo lsof -nP -i TCP:143 -sTCP:LISTEN
# Windows (PowerShell)
Get-NetTCPConnection -LocalPort 143 -State ListenOpen or block port 143 in a firewall
# Ubuntu / Debian (ufw)
sudo ufw allow 143/tcp
sudo ufw deny 143/tcp
# Windows (PowerShell, as administrator)
New-NetFirewallRule -DisplayName "Port 143" -Direction Inbound -Protocol TCP -LocalPort 143 -Action AllowOn cloud servers, also open the port in the provider’s security group or network firewall. At home, a service behind a router needs a port-forwarding rule to the device’s local IP.
Test it from outside
A port can be open on the server but blocked upstream. The Port Checker connects from the internet, so it shows what the world sees. From your own machine, use nc -vz example.com 143 (macOS/Linux) or Test-NetConnection example.com -Port 143 (Windows).
Port CheckerTest whether a server accepts connections on specific ports — web (80/443), mail (25/587/993), SSH, databases or remote desktop — and see which are open, closed or filtered by a firewall.
Open the tool Frequently asked questions
What is port 143?
Internet Message Access Protocol: email clients read and sync mail on the server.
Is it safe to open port 143?
Open only if you need it. Expose it only when required, with encryption and strong authentication, ideally limited to known IP addresses.
How do I check if port 143 is open?
Enter your domain or public IP in the Port Checker with port 143. It tries a TCP connection from the internet and reports open, closed or filtered.