[imp] forcing reply to as from

[Sean Meacher]

Setup:
Horde 2.0
IMP 3.0
Turba 1.0

Basically there's a shared server with a few hundred domains on it. One of
these has a large number of users whose email addresses are
Firstname.lastname at example.com with a username of examplexxx where xxx is
a number. 

The issue is that once the user has logged in and set their 'full name' &
'reply to' address to the format above, emails show up (in outlook) as
being from:
Firstname Lastname <examplexxx at example.com>
which some people see as a security risk as it shows their username.
The 'reply to' address does work, but my main question is:

Is is possible to have the 'reply to' address (which will be
<firstname.lastname at example.com> ) displayed rather than
<examplexxx at example.com> ?
So emails show up as:
"Firstname Lastname <firstname.lastname at example.com>"
(override the shown from address with the reply to)

And can I do this through the IMP code rather than hacking about with the
mailserver (sendmail 8.9.3)? If so, how?

Secondly, I was wondering if it would be possible to make this change just
for this domain (like if $domain=="example.com" do this ) as this is only
really an issue with this customer.

TIA,
Sean Meacher
Hotlinks Internet Services