[exim] Set domain on outgoing mail for one user only

Hi exim users ...

We have exim on a Debian virtual private server.
Can anyone suggest how we would configure exim
to set the domain on outgoing email, but for only
one user?

Eg: like the qualify_domain option but for one
user rather than all?

I'm guessing this is really a very obvious and
simple thing to do.

Thanks ...

:::: R ::::