-Go to your PBX's Settings > Email > SMTP server. Verify that these settings are correct and that the "Test" button works and that you receive a test e-mail when pressed.
-Verify that the extension for which you're having a meeting for actually has an e-mail and that this e-mail is correct.
-Send Welcome e-mails, verify that these actually go through to the expected e-mails.
-Make sure the PBX has port 5001 open to the outside world
-Make sure that you have a valid certificate for the PBX. Expired or self signed certificates are invalid.
Once the above conditions are met, you'l be able to receive and send e-mails in general on your PBX, not only for WebMeeting.