Use MX record when sending to $mydomain

[cont...@rusanu.com]

Hello,

I'm using sendmail to send to "someu...@mydomain.com" from the machine  
that is the A DNS of mydomain, but is not the MX one. postfix relays  
this to 127.0.0.1 and it gets rejected with 450 user unknown. I want  
it to use the normal rules and deliver it to the MX registered address  
for "mydomain.com", where my pop box is.

I looked over the main.cf but nothing jumped out 'fix me here' to  
solve this. Any hint?

Thanks,
~ Remus