I have a group "allusers" that I use for blast emails. Last week I deleted a mailbox and now when I send an email message to the group I am getting an undeliverable error to that mailbox. I looked at the group and do not see that mailbox.
To fix this I recreated the user. when I went to the group his name was checked even though I did not add him. I unchecked, re-deleted the user and all is well now.