Hi Don,
Thank you for providing the NDR.
The error message #550 5.1.1 may be caused by the following conditions:
1. The recipient email address was entered incorrectly by the sender.
2. No recipient exists in the destination email system.
3. The recipient mailbox has been moved and the Outlook recipient cache on the sender's computer has not updated.
4. An invalid legacy domain name (DN) exists for the recipient mailbox Active Directory.
For this scenario, please make sure the email address dcooley@Bdomain.com exists.
You can connect Windows PowerShell and type the following cmdlet:
Get-Mailbox -Identity dcooley@Bdomain.com
If the mailbox exists, this cmdlet will show the correct information.
Thanks,
Bourne Zhang