Hi lohplt,
Based on your description, I understand that you can't remove your customer domain.
To troubleshoot this issue, I suggest you do the following steps:
1. Check and remove your customer domain name from your company’s primary domain:
1) On the Admin page, in the top right corner, click your company name.
2) Click Edit.
3) Check the domain under the "Primary verified domain". If the content indicates that the domain is your custom domain, please change the domain name to the Office 365 domain.
2. Make sure you haven’t created any distribution groups with your customer domain name. To do this, in the portal, click Admin, click Exchange, in the recipients panel, click Groups.
For more information, you can refer to Remove a domain.
@Mch thanks for your useful information.
Best regards,
Jerry Cen