The  pipe(8)  daemon  processes  requests from the Postfix
   queue manager to deliver messages  to  external  commands.

virtual_transport : myowncustomizedtransport

myowncustomizedtransport  unix  -       n       n       -       -       pipe flags=DRhu user=vmail argv=/usr/bin/myapplication with some arguments (read man pipe to get a list of arguments you could get like $recipient and $client_address or $size)