Port 143: IMAP
Port 143 is IMAP, used by email apps to read mail stored on a server.
What runs on port 143
IMAP servers listen on TCP 143 and let mail clients read, sort and sync messages while keeping them on the server. The connection starts unencrypted and should upgrade with STARTTLS. The encrypted-from-the-start version runs on 993, which most providers now prefer.
On a home network
Home devices do not listen on 143 unless you run a mail server on a NAS or home server. Your phone and laptop connect out to a provider on 993 or 143.
If port 143 is reachable from the internet
On a mail server, IMAP being reachable is normal. If the server allows logins without TLS, passwords cross the internet in plain text, and IMAP logins are a common target for password spraying. At home, nothing should answer on 143 unless you host your own mail.
How to check whether it is open on your network
If you run a mail server, check that it requires TLS and consider closing 143 in favour of 993. An outside scan of your public address shows whether 143 answers.
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