The address lists page has an example listing on the right with all allowed formats.
e.g.: user@example.com, user@, @example.com, @.example.com, @[1.2.3.4]
So @example.com would be the complete domain and @.example.com would be all sub-domains for example.com.
Clicking on Help and Support -> Online Help on the top right while on a page explains a lot about the page for reference.
Regards,
Libin Varghese