[Exim] rewriting Sender: that doesn't have a domain

As some of you probably know, some versions of netscape put in a Sender
field without a domain, as in:
Sender: user
This not only breaks the RFC, but causes some sites to (rightfully) bounce
the Email

I'm trying to rewrite this as Sender: user@mydomain

While I am very familiar with regular expressions (shell or perl), I'm not
too familiar with what exim expects yet.

I got this one to work (according to exim -brw):
^([^@]+)                        $1@mydomain          s
(it doesn't work if I add a trailing '$' though)

but if I telnet to port 25, send an Email with Sender: foo, it doesn't get
rewritten :-(

What I am missing?

Thanks,
Marc
--
Microsoft is to software what McDonalds is to gourmet cooking

Home page: http://marc.merlins.org/ (friendly to non IE browsers)
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