We are evaluating Email to Case (On Demand).   It looks like the way it works out of the box is when we forward emails from support@company.com to the SF routing address, because the FromAddress for all the forwards is set to support@company.com, then all cases created via Email to Case (On Demand) will automatically have the Contact field set to a contact record for support@company.com (instead of the actual customer contact account for the person who sent the original email to support@company.com).  

What is the recommended (best practice) way to tackle this without having to write something like an after Insert case trigger to update the Contact relationship and parsing the TextBody of the email from the EmailMessage object to determine the true "sender"'s email address?

thx!