CRM Quick Campaign Email Sitting in 'Pending Send' with outlook client open

I was tearing my hair out with quick campaigns not sending emails. The Outlook clients were open with the correct user settings both in Outlook and the CRM user settings on the server. But the emails remained in a 'pending send' state. Restarted the Async service didn't resolve it either.
Finally, by chance, I deleted any quick campaigns that had emails with the 'failed status' and all the emails came to life and were 'sent'.

Strange but true.