I wrote a NET 4.5 client application tool using an IMAP require and a SMTP send command.
Now my current issue is I get the following exception at one client:
the remote certificate is invalid according to the validation procedure
I changed the code to:
ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };
So all certificates should be accepted. The server needs SSL. The same connection information is working with outlook or thunderbird without errors.
It could be something with trust issues like missing root CA or intermediate on client/server.
These validation issues can be logged (if you enable it) to CAPI2 event log. This answer might help you.