Mac crashing

iMac running 10.6.8, need help with We’ve been on O365 for over a year with no issues. Yesterday, on starting up it says "Address Book can’t verify the identity of “(our old on-prem Exchange)”. Then crashes. I deleted \Library\Preferences\ then restarted Mail; it prompted for mail account info. I gave it Office 365 credentials and it worked fine. Today it’s crashing again. When I delete the plist file today, it prompts for credentials, but then gives the same “Address book can’t verify identity…” error and crashes.

How can I make it stop looking for the Address book on the old on-prem Exchange? Or is there some other fix you’ve seen?