Exchange POP/IMAP Problems - no TLS Certificate

Check your Service State

[PS] C:\Windows\system32>Get-ServerComponentState -Identity YourMailServer
 

Server Component State
 ------ --------- -----
 [...]
 YourMailServer ImapProxy Inactive

 YourMailServer OabProxy Active

 YourMailServer OwaProxy Active

 YourMailServer PopProxy Inactive
[...]

The Proxy-Componentx are disabled.
So you can not connect and the server refuses “external” connections.

To fix this:
Open an ExchangeShell as Administrator

[PS] C:\Windows\system32>Set-ServerComponentState -Identity MAIL02 -Component PopProxy -Requester HealthAPI -State Active

SSL Check for IMAP/POP3/SMTP

IMAP via SSL uses 993

openssl s_client -showcerts -connect mail.example.com:993

POP3 via SSL uses 995

openssl s_client -showcerts -connect mail.example.com:995