I have a feeling the problem with SSL is not related to trust but perhaps it is using an older algorithm/cipher, which is disabled in Xeams for security reasons.

I also think that the server supports STARTTLS on port 143, indicating clients that they can upgrade their non-SSL connections to SSL. Xeams tries to upgrade to encryption on port 143 and fails because of the reason mentioned above. To confirm this theory try the following:

• Using telnet, connect to your IMAP server. The command is:
  
  telnet your.imaps.server.host 143
• Type this command once connected:
  
  abc CAPABILITY
• The server will return a list of supported features. If you see STARTTLS as a feature, my theory is correct.
• In that case, see if you can disable STARTTLS on your IMAP server. Once STARTTLS is disabled, Xeams won't upgrade to SSL on port 143.