When I read the spec the push support was ungood.

What it should do: Tell the server an XMPP or HTTP endpoint to which the server should push a specificed opaque blob when new mail arrives, and possibly other opaque blobs corresponding to other changes, and a (non-binding) rate limit.

With that, it would be easy to write a low-overhead server to support a particular mobile client. Accepts opaque blobs, decodes a little, sends blob to phone via giantmobilevendor.com (which also uses XMPP/HTTP), needs to store no state.