I hope this helps someone.
For the last two days, emails have been stuck in my Outlook (Classic) OutBox. When I do a Send/Receive, it never ends. Some Send/Receive tasks complete but many are stuck in Processing step. If I end the Outlook user interface, the task is still visible in Task Manager. Besides a couple of MS Exchange accounts, I have several POP/SMTP accounts and several IMAP/SMTP accounts.
I found that these problems were all caused by new Outlook Add-Ons automatically added by Microsoft to my Microsoft 365 Office Outlook (Classic) configuration. In the Outlook app, click File, then click Manage Add-Ons to view your Add-Ins. I found three Microsoft Add-Ons had been added to my Outlook configuration. I removed them all and restarted Outlook and all is working fine now.
Outlook Version 2408 (Build 17928.20114 Click-to-Run)