Relaying mail FROM: a specific domain only

Hello,

I sent a similar post a few days ago but I need more specifics.

I have a mail server that has problems with its IP address so I need to f=
orward
all mail coming from ( @mydomain.com ONLY no other domain ) to another
mailserver we will call: 123.123.123.123 [mail.externalserver.com]

@mydomain.com relayed for delivery to 123.123.123.123 [mail.externalserve=
r.com]

What configs do I set in main.cf for the @mydomain.com POSTFIX
What configs do I set in main.cf for the relay server?

This is what I have so far...

On the @mydomain.com server set relayhost =3D [mail.externalserver.com]
But, how do I ONLY relay mail from the domain @mydomain.com ??? and no o=
ther
domains this mail server handles?

On the RELAY SERVER 123.123.123.123 [mail.externalserver.com ]
what do I set in main.cf ? So far I believe I add the IP Address for
@mydomain.com to mydestination =3D mydomain.com, (plus all the other
mydestinations.. )

Is this correct?

thanks for anyone that helps!

regards,
matt