This seemed to already be technically supported but the validation is now stricter and the admin is more helpful:

* Postfix seems to allow @domain.tld as an alias destination address but only if it is the only destination address (see the virtual man page).
 * Allow @domain.tld if it is the whole destination address string.
 * Otherwise, do not allow email addresses without local parts in the destination.
* In the admin, add a third tab for making it clear how to add a domain alias.

closes #265