When running a workflow that contains a Send Notification action, notification emails are not being sent as expected.
Ensure the Web Front End (WFE) server can connect to the Exchange server over port 25.
This can be verified by running the command from the Web Front End server:
telnet <exchange servername> 25
If this fails, then it indicates that the (WFE) cannot connect to the Exchange server on port 25. The next steps would be to verify if the firewall on either the WFE or Exchange server is blocking port 25 or that the Exchange does not permit connections from the WFE.