Hi KMR,
As I understand it, your custom domain was used to BPOS before being used to Office 365 for Small Businesses. However, you cannot use the domain to send mails to external users because you would receive "550 5.4.1 Relay Access Denied"No-delivery Reports(NDRs).
To troubleshoot this error, generally we suggest trying the following steps:1. Connect Windows PowerShell to the Service, for the detailed instructions you can refer to: help.outlook.com/.../cc546278.aspx
2. Type the two following lines:
Set-AcceptedDomain <domain> -OutboundOnly $true
Set-AcceptedDomain <domain> -OutboundOnly $false
Note: Replace <domain> with the vanity domain that you want to use. You could also refer to the KB below for more detailed information: support.microsoft.com/.../2603474
3. One similar post you can refer to ishttp://community.office365.com/en-us/f/160/t/18498.aspx .
If the issue persists, please provide the following things for further troubleshooting.
1. Couldn’t you send mails to all external users, or just especial one?
2. Try to send a test email by using the contoso.onmicrosoft.com domain, and then let us know could the mails be delivered to the recipient.
Thanks,
Maggie Li