Symptoms
When trying to execute an email event the following error was captured:
The e-mail address [user]@[domain] does not have send on behalf of permissions on Exchange.
Diagnoses
The 'send on behalf of' permissions was a bit misleading here as the real issue was that that the '.com' portion of an otherwise good and valid email address had been dropped within the default String Table.
Resolution
Update the Sting Table to include the '.com'. This enabled the workflow to fir email events properly.