Hey Lucas 👋 Jacques here from Support Engineering, happy to help!
To set up a custom domain for outbound emails in Intercom, the domain owner does not need to be part of your Intercom workspace. You can use the domain of the emails that admins have signed up with, or the domains of Custom Sender Addresses. The domain owner will need to verify the domain to authenticate it for use with Intercom. This involves adding DNS records provided by Intercom to their domain's DNS settings.
Once the DNS records are added and verified, you can send emails from Intercom using the customer's domain. However, note that the Custom Sender Address associated with the customer's domain must be verified first. If the you’re is having trouble with the verification process, make sure they have followed the steps outlined in the our documentation, which includes verifying the domain with a DKIM record.
If your customer is still asked to create an Intercom account during the verification process, it might be worth it for you to reach out to the technical support team for further assistance, as they will be able to dig into the issue for you.