When comparing Wanderlust vs Mu4e, the Slant community recommends Mu4e for most people. In the question“What are the best email clients for emacs?” Mu4e is ranked 1st while Wanderlust is ranked 2nd. The most important reason people chose Mu4e is:
The UI is designed to allow for quick keystrokes to get around and is pretty clean. Header View: !(http://www.djcbsoftware.nl/code/mu/mu4e-2.png) Message View: !(http://www.djcbsoftware.nl/code/mu/mu4e-3.png)
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Pro Fantastic IMAP support
The key differentiator for Wanderlust is it's reliable and fast IMAP support.
It also supports a wide range of other protocols:
Pro Integrates nicely with existing Emacs packages
I'm currently using it with BBDB (for keeping my address book), Flyspell (spell checking as you type), Mailcrypt (digital signatures and encryption, here's my public key btw).
Can be integrated with Bogofilter, SpamAssassin, and probably whatever you want for spam filtering.
Pro Customizable with Lisp
The implementation is in elisp only, but allows you to customize the client in any way you wish.
Pro Nice UI
Has enough UI polish to make day-to-day use bearable. Shows nested folders, threaded conversations and Face/X-Face headers etc. It also has decent keybindings.
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.
Con Freezes Emacs when checking for new mail
Because Elisp is not a multithreaded language, it kind of freezes while checking for new mail. This can be bad if you're using the same Emacs instance for other purposes, like, writing code. :-) Not the case for me—I don't mind starting a new Emacs instance especially for WL; until one week ago I was using Thunderbird, which needs tons of RAM. Emacs is a lot lighter.