Good article from the founder of reMail (recently acquired by Google), I particularly like point #1:
I would aim for a RESTful design with the following properties: 1. All communication over HTTP / HTTPS: Pure TCP connections are great, but for transferring large amounts of email, HTTP is the way to go. Problems like security, parallel downloads, persistent connections, caching, compression, download continuation via ranges, and so on have already been solved. There is no reason to solve them again.
via Gabor hits Send: How to Replace IMAP.
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