According to the log, the POP3 server sends the following unencrypted error message when the client connects to it and attempts to negotiate a TLS session: "ERR Connection is closed. 13"
This is a server-side issue - the POP3 server is simply not willing to negotiate a TLS/SSL session and rejects POP3 clients immediately. The server log should contain more information about this. The issue seems to be similar to this one, which was caused by Microsoft Exchange server with improperly configured certificates.
