Port 143 (IMAP)

Internet Message Access Protocol for reading mail on the server, upgraded with STARTTLS.

Well-known (system) port, 0-1023 TCP Email Expose only to trusted sources

What port 143 is used for

Internet Message Access Protocol for reading mail on the server, upgraded with STARTTLS.

Expose only to trusted sources. Prefer 993. If 143 stays open, require STARTTLS before LOGIN so passwords never travel in clear text.

Service nameProtoDescriptionAssigneeReference

imap TCP Internet Message Access Protocol IESG RFC 3501, RFC 9051

Registry dates for this port: last modified 2021-02-22.

Linux: is anything listening on TCP 143?

sudo ss -tlnp 'sport = :143'

Linux / macOS: which process owns the port

sudo lsof -nP -i :143

Older Linux / BSD (net-tools)

sudo netstat -tlnp | grep ':143 '

Windows (Command Prompt)

netstat -ano | findstr :143

Windows (PowerShell)

Get-NetTCPConnection -LocalPort 143 -State Listen | Select LocalAddress,OwningProcess

From another machine

nc -vz -w 5 HOST 143

From Windows

Test-NetConnection HOST -Port 143

Replace 203.0.113.10 with the address or network you trust. With ufw, make sure the default incoming policy is deny.

ufw (one trusted source)

sudo ufw allow from 203.0.113.10 to any port 143 proto tcp

iptables

sudo iptables -A INPUT -p tcp --dport 143 -s 203.0.113.10 -j ACCEPT
sudo iptables -A INPUT -p tcp --dport 143 -j DROP

nftables

sudo nft add rule inet filter input ip saddr 203.0.113.10 tcp dport 143 accept
sudo nft add rule inet filter input tcp dport 143 drop

Windows Firewall

New-NetFirewallRule -DisplayName 'TCP 143 trusted' -Direction Inbound -Protocol TCP -LocalPort 143 -RemoteAddress 203.0.113.10 -Action Allow

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Assignment data: IANA port number registry (public domain), retrieved 2026-11-10.