Google certainly has a bunch of extensions in IMAP. I'm not sure about the messages themselves, but I wouldn't be surprised if Google has extra metadata and su

Google certainly has a bunch of extensions in IMAP. I'm not sure about the messages themselves, but I wouldn't be surprised if Google has extra metadata and supports features that only work within the world of GMail and gracefully degrade in other clients—but, unlike IMAP, I haven't worked with that directly, so I'm not confident either way. (Concrete example: GMail has additional integration with Google Calendar just like Outlook has integration with Outlook's calendar, but I'm not sure how that manifests at the protocol level.)