Resolution: If TIBCO Nimbus is failing to send email notifications, and you receive an error similar to "Cannot connect to server (Already connected.)" within the logs for the email service ( \\Nimbus\Action Server\Logs\Email) - This will be caused by the port for SMTP traffic being blocked between the Nimbus server and SMTP server. Particularly if the SMTP server is externally hosted.
You should test this using TELNET to confirm there is a block of traffic via this port between the two servers. Once resolved recheck the email log and confirm the errors have stopped occurring.