[BlueOnyx:02876] how to receive an error msg "No such user here"

[Eiji Hamano (bluequartz)]

Dear

We are using "email aliases" value on "email address field" of his mailer.
For example;  real acount "h0001" has an aliases name "support".
He sets "support at mydomain.com" as email address,  not "h0001 at mydomain.com".

However when he sets the "support at mydomain.com",
he cannot receive the error msg like as follow.

   ----- The following addresses had permanent fatal errors -----
   <qqq at xxx.yyy>
   (reason: 553 5.3.0 <qqq at xxx.yyy>... No such user here)

               "qqq at xxx.yyy": is a mistaken terget address.


Only when he sets "h0001 at mydomain.com", he can receive the error msg.

How do we receive the error msg with aliases address "support at mydomain.com" 
?
We don't want to tell receivers about real account.

Eiji Hamano