I hate to say it but that hostgator response is wrong. You can create a mail service for a specific customer and bind it to their static ip. But, I have a static ip for my site and i have been PR 3 then 0 then 3 then 0 again so im about to contact google and see what the issue is. Beware of link farms as that can get your site blacklisted in regards to PR and I think that is what happened with mine.