Hi VDP,
I have analyzed the NDR you provided in private message.
To narrow down this issue, I’d like to collect some detailed information.
1. Was your domain virtualXXXXXX.org once verified in other email service, such as Google Apps?
2. What’s the type of the mailbox vdp@.........? user mailbox, shared mailbox, or distribution group?
3. Have you ever deleted this account and re-create it?
Based on the NDR you provided, I’d like suggest you sign in Microsoft Online Portal (MOP), please confirm this mailbox vdp@......... exists on the mailbox list at Exchange Control Panel (ECP). You can also connect PowerShell to Exchange Online, and then run get-mailbox to if you can get this mailbox in the result.
Thanks,
Young Yang