Hi @Huw Werrett ，
Welcome to our forum!
As far as I know, when you delete or move a large number of messages at once, Outlook can appear to freeze, but really, it's working behind the scenes. Check the status bar at the bottom of the screen. If Outlook is performing some operation on hundreds or thousands of messages at once, it might appear to be stuck. Give Outlook a few minutes to finish the process and it should start to work again.
If Outlook stuck at processing, try to use the Task Manager to force quit Outlook, follow the steps.
- Open the Task Manager by pressing CTRL+SHIFT+ESC keys at the same time.
- Right-click Outlook and in the new window that opens, click End task to force-quit Microsoft Outlook
Besides, Outlook works best when it's up to date. We recommend setting Windows Update to automatically download and install recommended updates. Bug fixes, new features, and performance enhancements are released regularly. To install the latest Windows and Office updates, see Install Office updates.
If the issue continues, try creating a new profile and connect your account to see if it makes difference.
To create a new profile: Go to Control Panel > Mail > Show Profiles > Add
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