We have GP 2013 configured to Outlook and the email goes to the Outlook outbox and sends, but doesn't go through -- instead the system admin sends back the email as undeliverable, saying:
Your message did not reach some or all of the intended recipients.
This message could not be sent. Try sending the message again later, or contact your network administrator. The client operation failed. Error is [0x80070057-0x80070057-0x000501].