This should be the troublemaker. By default, Discourse uses a secure connection, and since the certificate is not trusted, this won’t work.
I don’t think you can easily trust your certificate. You might be able to make it work by using an unencrypted connection (look for the pop3 polling ssl setting), but I wouldn’t do that unless the connection is to localhost.
The best solution might be to get a (possibly free) signed certificate.