[TurboFool]

As far as I'm aware you just need to edit the e-mail address field in the setup of the account and that becomes the reply-to address.

Out of curiosity, have you considered using Google Hosted Services for your e-mail on your domain? I used to use my web host's e-mail functionality, but then they added an easy way to switch over to Gmail for the e-mail (it's not hard to do it without help, but that was my motivation), and it's been great. My site's still hosted through my provider, but the domain MX records point to Google, so now I use the Gmail web interface for all the mail, and I have their full IMAP support. And doesn't require me to do anything fancy with reply-to fields. Might be worth looking into.