It sounds like you are possibly getting this with meeting requests you are seinding, or is it for emails other than meeting requests? Typically I see this type of behavior when you send a meeting notice to someone who has the deleted account in their delegates list still. They need to remove the account from their delegates. If this is for emails you send which are not meeting requests, that’s more difficult to figure out. Can you give more specifics please? If you are sending to a distribution list you have created you need to check it for the old address and delete the address from your list.