Hi !
 
I have just mentioned a behaviour of postfix handling catchall-addresses
on the domain on which postfix is configure.
Perhaps the solution is something you should integrate into the who-to.
 
try this:
my domain: "somedomain.de"
MAIN.CF:
--------
myhostname=somedomain.de
mydestination = mysql:/etc/postfix/mysql-mydestination.cf, somedomain.de
 
Setup up in Web-cyradm the domain "somedomain.de"
and do an catchall for one address of the domain.
 
-> RESULT
ALL mail - also for other domains on the same server - goes to the
catchall of "somedomain.de"
 
SOLUTION: (I guess you know it already ;-)
---------
Use an existing subdomain for hostname, e.g. mail.:

MAIN.CF:
--------
myhostname=mail.somedomain.de
mydestination = mysql:/etc/postfix/mysql-mydestination.cf, mail.somedomain.de
 
Regards,
Sebastian