Define root's email from address

I have some scripts which use the "mail" program to email a remote email address. The problem I am having is that the upstream mail server we send through isa bouncing these mails back as undeliverable as the senders address is not resolving. The error is similar to: ... Domain of sender address root@store.atlantic does not exist (in reply to MAIL FROM command) How do I setup root's senders address to be something like root@myvalidinternetdomain.com ? I assume it is all down to /root/.mailrc, but I am struggling to find the appropriate syntax for doing this. Any ideas ? Richard