As a marketing tool we're sending email on behalf of numerous clients and their domains. Some of those clients are either using Mailgun directly or through another marketing service.

This causes problems when they attempt to validate their DKIM and SPF records, as we can't add their domain to our account.

Adding a sub-domain instead is a work-around at best and transferring the domain to our account is impossible most times, so being able to share domains across Mailgun accounts would fix this nicely.