Using older versions of Outlook for Mac for personal email and calendar
Hi WDM1,
Thank you for choosing Microsoft community.
Based on your description, I tried to test from my side. For the classic/old Outlook for Mac, creating rules for an IMAP account is feasible. But if you're using the New Outlook for Mac, according to Create a rule in Outlook for Mac - Office Support (microsoft.com), "in the new Outlook for Mac (as of October 2020), only server rules are available, client rules are no longer supported".
If you're using the New Outlook for Mac to create client rules, we're sorry for any inconvenience caused. If you don't mind, please kindly click the toggle to switch to the classic/old Outlook for Mac. There you'll be able to set rules easily.
Hope it helps! Thank you.
Stay safe & Sincerely,
Qian