There's a great mail client called mahogany that also can read maildir folders. It's an open-source project which quite mature (Mahogany 0.66 was released on January 18, 2004) and active. It doesn't depend on any desktop environement like KMail does, and unlike Sylpheed-Claws, it can read AND write to maildir folders. However, an ebuild for portage is not available.
Here's the project page:
Steps to Reproduce:
1.emerge -s mahogany
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An ebuild that shows the size of the download.
If you (or someone else) will write an ebuild for Mahogany, I will gladly add it to Portage.
Besides, Sylpheed-Claws _can_ write to maildir folders. Just compile sylpheed-claws with maildir USE flag and portage will install maildir plugin.
Feel free to reopen once an ebuild is attached.
ebuild here: http://bugs.gentoo.org/show_bug.cgi?id=46896