Port closed for localhost but open for the remote connections

Question

When I try the folowing command from a remote machine it says port open

nmap myserver.com -p 143 

But when I try the same thing from a local machine (where the mailserver is installed) it says port closed.

nmap localhost -p 143 

UFW is inactive but when I activate it and add the 143 allow the result is the same.

Btw,

nmap localhost -p 80 says open

What am I missing?

Answer 1

You might try something like netstat -ln | grep 143 and pay attention to the IP it's listening on.

If it's listening on 0.0.0.0, there must be some packet filtering interfering.

But if it's a specific IP like 1.2.3.4, you know that's different from 127.0.0.1 (localhost), and that's why your server is not answering.

If you are using IPv6, the corresponging situation may apply.