Use personal aliases MTA option

Alias and address MTA options:  (0 or 1)
The  MTA option controls whether or not the MTA makes use of personal alias databases as a source of aliases for local addresses. The default is 1, which means that the MTA will check such databases, if they exist. A value of 0 will disable personal aliases and make them unavailable to all users.

See also:
 * Alias and address MTA options
 * Database MTA options