Problem:

When you are trying to delete a verified domain name in Office 365, an error pops up saying that some users or Office365 services are still attached to the domain.

Root Cause:

Just like what the error said, some of the Office 365 services or users are still attached/assigned to the domain name that you are trying to remove.

Solution:

Things to check:

1. Ensure that no users are associated with the domain that you are trying to delete. You can verify this by going into Users And Groups, and Edit a user. Ensure that the domain you are trying to delete, eg, abc.com, is not listed there.
2. Ensure that no security groups/distribution groups have the accounts attached to abc.com. Security groups/distribution groups can be access by logging into Office 365, click on Users And Groups, and click on Security Groups.
3. If you have just deleted the users, or changed the domain for each individual users, you will need to wait for a while (1 min?) as it will need to sync the changes to the different Office365 service settings.
4. If the accounts are uploaded to Dirsync, you will need to stop the Dirsync synchronization to change the accounts to a Cloud Only account. Then, you will need to do step 1-3 above to delete the Security groups; and/or manually modify the e-mail addresses fields in Office 365, Exchange Online.