Port 110: POP3
Port 110 is POP3, an older way for email apps to download mail from a server.
What runs on port 110
POP3 servers listen on TCP 110 and let a mail client download messages, traditionally deleting them from the server afterwards. The connection starts unencrypted and can upgrade with STARTTLS; the encrypted-from-the-start version is on 995. IMAP has largely replaced it because it keeps mail in sync across devices.
On a home network
Home devices do not listen on 110 unless you run a mail server. Older mail clients and some printers that fetch mail connect out to a provider on 110 or 995.
If port 110 is reachable from the internet
On a mail server, POP3 is expected, but plain 110 without enforced TLS sends passwords in clear text, and POP3 logins are a common target for password guessing. At home, nothing should answer on 110.
How to check whether it is open on your network
If you host mail, require TLS or close 110 in favour of 995. An outside scan of your public address shows whether 110 answers.
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