Deleting invitee contacts in iCal

Hello JB12inMI,

Thanks for using Apple Support Communities. It is expected behavior for Mail to keep a list of previous recipients in order to offer them as suggestions. It sounds like you've done a great job trying to resolve this issue. If you haven't already, take a look at the steps below to make sure you've followed the appropriate steps to remove incorrect addresses from the list:

1. "In the Mail app  on your Mac, choose Window > Previous Recipients.
2. Find the address you want to delete. You can search for it or sort the list by clicking a column header.
3. If you want to check the address in the Contacts app, double-click the card icon in front of the address. If a card doesn’t exist yet and you want to keep the address in Contacts, select the address, then click Add to Contacts. 
4. Click Remove From List."

Remove obsolete email addresses in Mail on Mac - Apple Support

If the issue persists, test in a safe mode. Starting in safe mode allows your Mac to run certain checks, remove caches, and prevent login items from automatically launching. This can isolate possible third party issues. Use the steps and information in the following resource for assistance with safe mode: How to use safe mode on your Mac - Apple Support

Let us know the results. Be sure to include any other steps that you may have tried.

Cheers!