[exim] send to smarthost with a unified MAIL FROM address

Hello list,

I'm a bit stuck and tired with this problem. Maybe I was choosing the wrong
path...

My problem is that our smarthost accepts mails from us only from a given IP
address (on which my exim instance runs), and with only a specific address
in the MAIL FROM: command. I was trying with rewrite rules:

*@* bocmok@???

with this, I succeeded, as mails went out from my box, but local mailing
failed, as exim sent thos mails to the smarthost for somewhat reason. So
sending a mail with "mail polesz@???" sent to the smarthost, so
as "mail polesz", which should arrive in the local user "polesz"'s mailbox.

Can you suggest me a new solution, or a correct rewrite line for this
problem please?

Thanks in advance,
Gergely Polonkai