Internet maill

Hi.
we are using Exchange 5.5 .my company has 2/3 branchs office . every site has exchange server all are connected by X.400 connector (WAN)

In Branch where i am working it sends there all internet mails to our headoffice which has RF link . also i am using ISDN line for intenret browsing purpose .

I wanted used during office hours (9 to 6)i am mail should go thro my local office ISDN line & after that it should go to my headoffice RF link
I don`t want to do any manual changes in the my EXCHANGE SERVER .. I tried with maill briage .. but not working Could any one has soluation on that ..