Re: When I set an account to aliasx@xs4allnl (that is possible), the emails sent to personx@xs4allnl are shown too.
And your point is that they should not (be shown)?
Aqua syncs whatever it finds on the mail server -- so if messages addressed to both aliasx@ and personx@ go into same "real" account, whatever it's called, and end up in same folder, then that what you'll get in Aqua Mail.