A little "issue" has been found and is going into further investigation.
If you have SSL enabled for services with Courier (Dovecot isn't impacted) and are running the newly Windows 10, at least on 3 tests PC (2 updated (Win7=>Win10 ; Win8.1=>Win10) and a fresh Win10 install), all are unable to connect to POP3/IMAP.
From Windows events manager, we get an error with SChannel, which suggest an Handshake failure.
One posibility could be the Cipher list ( @Nuxwin )
YouTrac ticket created, Ref : https://youtrack.i-mscp.net/issue/IP-1401
More information to come as we get them.