...

As a wild ass guess, I think maybe the client is using used "STARTTLS" intead of "SSL", or some other SSL requirement disagreement.

I believe it has more to do with the encryption method (plain, login, cram-md5) than with the TLS. The TLS encryption happens on the network layer whereas the encryption method is used at the IMAP level.

Those are authentication methods. However, Googling the heck out of the error message, it appears the OP's error message is a common problem fixed by resetting the PopProxy configuration

https://social.technet.microsoft.com/Forums/ie/en-US/40eb0164-0bcf-499a-9401-b6a688b5fcfd/log-onto-incoming-mail-server-pop3-your-server-does-not-support-the-connection-encryption-type?forum=exchangesvrsecuremessaging

https://blog.zubairalexander.com/outlook-error-0x800ccc1/

Joseph Tam <jtam.home@gmail.com>