Port 143 IMAP
Port 143 is the IANA-assigned imap port for Internet Message Access Protocol (TCP).
What is port 143 used for?
Port 143 is assigned to imap and uses TCP. The IANA registry describes the service as “Internet Message Access Protocol”. It sits in the well-known range, which operating systems reserve for core network services, so software expects it at this number by default.
Ports are the transport-layer address a service listens on. A client that wants to reach imap connects to port 143. If nothing is listening, the connection is refused. That is why a port conflict appears whenever two processes try to bind the same number.
Use the commands below to confirm whether port 143 is actually in use before you change firewall rules. Blocking the port stops new inbound traffic at the firewall, but it does not stop a local process from listening, and it does not remove the service from the machine.
Source: IANA Service Name and Transport Protocol Port Number Registry. Registered assignee: [IESG]. Reference: [RFC 3501]; [RFC 9051].
Check and block port 143
Copy-ready commands for the protocols assigned to this port. Run the check first, then block only what should not be reachable.
Check on Linux
sudo ss -tlpn 'sport = :143'Check on Windows
Get-NetTCPConnection -LocalPort 143 -State Listen -ErrorAction SilentlyContinueBlock with UFW
sudo ufw deny 143/tcpBlock with iptables
sudo iptables -A INPUT -p tcp --dport 143 -j DROPThese commands are examples for a typical host. Check the listener before you change firewall rules, and persist iptables rules with iptables-save or netfilter-persistent. On Windows, use Windows Defender Firewall or New-NetFirewallRule for persistent blocks. The iptables rules here cover IPv4; mirror them with ip6tables if IPv6 is enabled.
Is port 143 safe to expose?
IMAP sends credentials and mail in clear text unless upgraded with TLS. Prefer IMAPS on 993 and limit access to trusted networks. Expose this port only when the service is actually needed. Keep it patched, prefer an encrypted alternative, and limit source addresses with a firewall allowlist.
Where port 143 is usually used
Most traffic on port 143 falls into these scenarios. The demo below is a safe way to confirm the listener before you change anything.
- Mail clients sending messages through a relay or fetching them from a mailbox.
- Mail servers exchanging messages for a domain or syncing folders with clients.
- Deliverability, authentication and TLS tests that connect directly to the mail service.
Test the IMAP listener
Start TLS over the plain IMAP port to inspect the certificate and banner.
openssl s_client -connect mail.example.com:143 -starttls imapCommon problems with port 143
Connection refused or timed out
Refused usually means the host is reachable but nothing is listening on port 143. A timeout more often means a firewall drop, a wrong host or a service bound to a different interface.
Port already in use
Find the process holding the port with sudo ss -tlpn 'sport = :143' or sudo lsof -i :143. Stop that process or configure it to bind another port; killing it without identifying it can take down a production service.
Firewall rule did not survive a reboot
UFW and Windows Firewall keep rules across restarts. Raw iptables rules do not: save them with iptables-save or the netfilter-persistent package, and re-apply after upgrades that reset the ruleset.
The port is blocked but still reachable
A firewall rule filters inbound traffic; it does not close the listener. Confirm with the check command above and bind the service to 127.0.0.1 when only local clients need it.
Port 143 at a glance
| Port number | 143 |
|---|---|
| Service name | imap |
| Transport protocol | TCP |
| Category | |
| Range | Well-known port (0–1023) |
| Risk level | Medium |
| Registered assignee | [IESG] |
| Reference | [RFC 3501]; [RFC 9051] |
| Registry description | Internet Message Access Protocol |
| Source | IANA Service Name and Transport Protocol Port Number Registry |
Frequently asked questions
What is port 143 used for?
Port 143 (IMAP) uses TCP. Internet Message Access Protocol.
How do I check if port 143 is open?
On Linux run `sudo ss -tlpn 'sport = :143'`. On Windows run `Get-NetTCPConnection -LocalPort 143 -State Listen -ErrorAction SilentlyContinue` in PowerShell. If the port is open, the output shows a process listening on port 143; empty output usually means nothing is bound to it.
How do I block port 143?
With UFW run `sudo ufw deny 143/tcp`. With iptables run `sudo iptables -A INPUT -p tcp --dport 143 -j DROP`. Persist the iptables rules with your distribution's netfilter-persistent or iptables-save tooling so they survive a reboot.
Is port 143 safe to expose?
IMAP sends credentials and mail in clear text unless upgraded with TLS. Prefer IMAPS on 993 and limit access to trusted networks. Nevertheless, expose this port only when the service is actually needed. Keep it patched, prefer an encrypted alternative, and limit source addresses with a firewall allowlist.
Related ports
Primary data: IANA Service Name and Transport Protocol Port Number Registry.
Last reviewed 2026-10-30 · Methodology · About · Privacy