Hello Jim,
According to the description, you can refer to the following steps to troubleshoot the issue:
1.Make sure the domain is not set to be the default one.
2.Remove other e-mail address or SMTP address references in Users.
3.Remove other e-mail address or SMTP address references in Groups.
Here is a knowledge base (KB) article for your reference:
You receive an error message when you try to remove a domain from Office 365
Besides, there is another thread in the forum with the same issue which you can refer to:
http://community.office365.com/en-us/forums/146/p/56712/206221.aspx
For the steps above, you can also follow this article: http://www.configureoffice365.com/remove-office-365-domain
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Regards,
Cherry Wang