[tlug] Postfix and relaying

[Gerald Naughton <naughton123@example.com>]

Hi All
Wondering if anyone has this experienced

Currently i host 2 domains in japan and everything works find

   xxxx.jp
   yyyy.co.jp

Now my headoffice wants to route all emails thru
   headoffice.com

So for postfix i would do 
  relayhost = [headoffice smtp ip]

But i only want to want internal email by sender headoffice.com
thru this relayhost 

Guessing it is something to do with
transport_maps = hash:/etc/postfix/transport

So i want the sender
  sender_username@example.com -> HeadOffice Mail Server

Otherwise
 sender_username@example.com
 sender_username@example.com  ->  Directly to Person

any ideas/hints would be helpful ?
thank in advance
gerald