No mail domain for subdomain
Requirement: Activate "Use the domain limits in client module to add new domains"
Reproduce: Log in as admin Add example.tld to the clients domain list. Log in as client. Create new domain (=selecting): example.tld Create subdomain or even vhost subdomain (if activated): sub.example.tld
Now try to create an email address email@example.com This won't work because you cannot create (= select) an email sub-domain as the list is filled only by the given main-domains