Hi ITData,
From your description and action, the user’s mailbox was deleted successfully. I try to reproduce the issue on my side, based on my test, after deleting the user, it was removed from distribution group and I can send email to this group normally. Regarding your issue, I would like to more information to narrow down the issue.
1. How do you delete the user mailbox? I mean you did it by using PowerShell or delete it in exchange control panel.
2. You also mentioned that group member received a message that the message was not delivered. Could you please tell me more about this, is there any detailed information on this message?
Please post back if you have any updates, I’ll do my best to help you on this.
Best Regards,
Tiny Duan