I have set up a number of worklflows (e.g. when a user edits a contact that is owned by another person, the owner receives an email notifying them that a change has been made to their contact). However, a large number of mails never reach the users, and are left in the workflow queue. A message appears that says This message has not been submitted for delivery. No other explanation is given.
Any ideas on where I should be looking to see why they are not being delivered?