[Exim] Exim Header - Hostname Problem

Hi All,

Recently it was brought to my attention (by a client) that when e-mail is
sent through exim, there is a slight discrepancy in the hostname that is
used in the mail header. We offer web hosting to numerous shared and
reseller clients. All shared clients are on a single IP address, and when
they send mail, the hostname is fine. However, reseller clients are given
two dedicated IP addresses (for their nameservers, where the first IP is
also used for them to host their shared clients) and when these reseller
clients (and their clients) send emails, the hostname continues to show up
as the hostname of the server as displayed by "uname" or "hostname."

I can only assume that this is being caused by the following directive in
exim.conf:

# Specify your host's canonical name here. This should normally be the fully
# qualified "official" name of your host. If this option is not set, the
# uname() function is called to obtain the name.

# primary_hostname =

The control panel software we use has exim preconfigured to work in the
shared environment we currently use it in, so making drastic changes to
exim.conf could potentially cause exim to not work properly for all of our
clients.

My question here is, since each reseller client is given two dedicated IP
addresses, is there anyway that Exim can resolve the hostname of the the IP
address being used to send the mail (that being the main IP address for the
reseller client). For example, if our business is eximhost.com, and the
machine's hostname is server1.eximhost.com it would be very appropriate for
outgoing mail to display server1.eximhost.com in the header (which it
currently does). Now, if a reseller client, for example eximhostreseller.com
has a hostname (on the same server) of ns1.eximhostreseller.com it would NOT
be appropriate for outgoing mail to display server1.eximhost.com in the
header - it would be appropriate for it to show ns1.eximhostreseller.com.

I am not looking to completely ignore sending the hostname in the header,
I'm simply looking for a way to have exim properly fetch the hostname/IP of
the account mail is being sent from.

Any help on this matter would be greatly appreciated! Thanks for taking the
time to read this!

- Matt