Hi Limeyguy, I think the pro version doesn’t affect the email functionality; it should work well in both. Nice that you support the development work of the EM team by purchasing the pro version, though. We’ve done the same.
You state neither version sends mail “most of the time”. Could you specify a bit? Eg. do some types of emails always get sent while others do not? Are there some similarities with emails that get sent that are different from those that don’t?
For example one issue we encountered earlier with Google Calendar turned out to be related to special characters in the event fields (event name, description, location name) and that it affected only users who were not logged in to Google when clicking the Add to calendar link. This helped in identifying the source of the issue (unfortunately, Google product bug). If you can find information that similarly helps you find a repeatable way to produce the problem, or to avoid it, that helps greatly in locating the cause.