> All of those support IMAP currently, an open standard. Exchange has MAPI as well. Gmail supports IMAP, with custom extensions to support threading. As far as

> All of those support IMAP currently, an open standard. Exchange has MAPI as well. Gmail supports IMAP, with custom extensions to support threading. As far as I understand it they use a custom protocol for their own gmail apps because of IMAP's shortcomings. Its basically impossible to build a modern email app directly on top of gmail's public APIs. You can do it - but only by mirroring and re-indexing all the email.