Thanks uxbod! I found the answer:
Set catchall:
su - zimbra
zmprov modifyAccount
user@domain.com zimbraMailCatchAllAddress @domain.com
Unset catchall:
zmprov modifyAccount
user@domain.com zimbraMailCatchallAddress ""
btw, it will be very helpful if all possible settings are made available on the GUI for the administrators without having to remember command lines.