I originally wanted to write that the "Could not create SSL/TLS secure channel" error message doesn't seem to come from Rebex libraries and ask for a communication log, but I noticed your update before posting that.
However, none of Rebex components actually uses .NET's
ServicePointManager - not even our multi-platform TLS/SSL library. I can say for sure that
GetMailMessage does not change anything in
Of course, our sample application uses Google's assemblies for OAuth. Those assemblies most likely uses .NET's
HttpWebRequest for HTTPS communication, and that relies on
ServicePointManager. However, if anything is changing its properties, it is almost certainly not a Rebex assembly.