Associate email to contacts, other properties

[Scott F.]

I am trying to allow users to associate an incoming email to a 'project'.
When an email comes into the user's Inbox, I need to be able to allow the user to associate this email to a project from a list of projects that I will populate, similar to how contacts can be associated to Categories on the default contacts form.
I have tried to create a custom form that has my dropdown list in it, but can't seem to get my custom form to be used when the email is opened
Anyone have an idea of the best way to go about doing this

Thanks.