Port 110 runs POP3: the client downloads messages and usually deletes them from the server. Encryption is only available via STLS (STARTTLS) after connecting.
RFC 8314 recommends implicit TLS, i.e. port 995. Once all clients use 995, close port 110 in the firewall. In Dovecot, ssl = required at least prevents plaintext logins.
Typical complaint: mail missing on a phone because another device fetched it via POP3 and deleted it. For multiple devices, IMAP is the right protocol.
Check
openssl s_client -starttls pop3 -connect mail.example.com:110