Starting with version 0.9.8, mu provides an emacs-based e-mail client which uses mu as its back-end: mu4e.
Through mu, mu4e sits on top of your Maildir (which you update with e.g. offlineimap or fetchmail). mu4e is designed to enable super-efficient handling of e-mail; searching, reading, replying, moving, deleting. The overall 'feel' is a bit of a mix of dired and Wanderlust.
Ranked in these QuestionsQuestion Ranking
Pro Can write rich text emails
Using the (beta) org mode, Mu4e can compose fully rich text emails.
Pro Built-in Autocomplete
Doesn't require you to import address books for autocomplete, learns from your emails just like Gmail.
Heavy actions never block emacs, unlike most other emacs email clients.
Pro Fully Search Based
Mu4e has no folders or other organizational tools, rather it relies on very fast search queries to help you find your mail.