Hi Leo,
Domain Admin's NEVER had impersonate abilities. Only System Admin accounts, can do this, you can add another system admin and not check the box to give them this permission.
If you login as System Admin then manage a domain, you will be able to impersonate users, but if you login with a User that has Administration rights for his Domain, you will not have the ability to impersonate a user. That would be a bug.
What SmarterMail version are you running? I tested this again with Version 15.4.6151 and cannot impersonate a user while logged in on the Domain Admin account.
-dave