1. This problem will be happening on OnPremies Exchange Server to Exchange Online or Existing OnPremies Exchange Server dead and new exchange server does not migrate/upgrade from old.
  2. Only internal organization address, not affect external address
  3. Because Exchange Online/OnPremies address store on Outlook Auto-Complete list store address is related to the server.

Solution 1:

Outlook File > Options > Mail > Send Message > Click “Empty Auto-Complete List”


Solution 2:

This solution already show on bounce back email