I switched between Mail.app (IMAP from my GMail account) and just plain Gmail, and what worked for me when using Gmail was using Mailplane.app - the idea is a desktop 'window' into the regular Gmail web interface. One of the things it does that helps it integrate with OF is that you can 'Copy Mailplane Mail URL' (or something to that effect) and then paste it into OF, which gives you a hyperlink that'll open Mailplane and navigate to that message.
http://mailplaneapp.com/beta/index.html
I ended up switching back to Mail.app with IMAP. Hope that helps!