postfix smarthost to relay in same domain

[str tux]

> if you mean, your old mailserver should forward outgoing mails to the new
> server see here:
> 
> http://www.postfix.org/BASIC_CONFIGURATION_README.html
> 
> see section  "What delivery method: direct or indirect"
> 
> hth
> Roger

Thanks Roger,

relayhost is properly configured and relaying works except as i
desired. limme explain further:

domino server users:
john at mydomain.com
peter at mydomain.com

postfix server users:
jane at mydomain.com
laine at mydomain.com

i have no issue in my domino server as anything not local user to the
domino server it is forwarded to my postfix server.

the problem now occurs in postfix.
if i put mydestination and relayhost as:
mydestination = $myhostname, localhost.$mydomain, localhost, $mydomain
relayhost = [mail.mydomain.com]

i'm not able to send to john at mydomain.com

if i set mydestination and relayhost as:
mydestination = $myhostname, localhost.$mydomain, localhost
relayhost = [mail.mydomain.com]

i'm able to send to john at mydomain.com but not able to receive email
within any users in postfix server.

in exim as i'm little bit familiar, you can set a transport smarthost
and you can set to check first any local users in the local server and
if it is not available it will be forwarded to the relayhost
(smarthost).

how to achive same thing in postfix?

str