In general IMAP means, you can sync several folders in opposite to POP, so imap is more powerful, especially, if you want to sync your outbox, I have not found out yet, how openERP respects standard folders as I use in Gmail or if I would have to start using a mailprogram like thunderbird or outlook, in order to have good integration and keep all my mail (response) record in one place. 

I believe that's true for IMAP, but my understanding of POP is that it grabs everything that is read or unread and removes it from the server (that's what POP implementations generally mean). I could be mistaken though, I basically avoided any serious POP usage.

For a little while I was working on an alternative IMAP reader for OpenERP that would check read and unread messages based on a rolling sync time. First it would grab all messages in the last 2 weeks, then every sync after that would grab every message from 1 minute before the last sync until the present, and make sure to skip overlapping messages.

Never finished it due to a lack of interest. We're all using Outlook already, so it wasn't really helping to basically duplicate our mail database in OpenERP. Plus, the email "client" is very lacking.