Aquamail does not distinguish between "the same" vs "different" domains.
So, you have a choice of either saving those addresses into "Contacts" or enabling the option in Aquamail to save all the addresses to which you've sent messages, -- in Contacts (automatically).
And there is no option to specify the "default" domain for your account that would be added to the addresses in the form "username" (i.e. to convert them automatically to "username@yourdomain.com").
I can see how that could be a handy feature for business accounts where majority of the correspondents are within the same domain.
