Comparison of existing email clients

For the next evolution of Mail2Voice, we need to start from a solid, proven and flexible existing client. To help deciding which software fits our needs the best, this page review as much FLOSS clients as possible.

Given our constraints, the review criteria will be the following:

Software License (GPL, MIT, etc.)
Confidence (low, medium, high): does the software seems to be reliable, maintained, etc?
Community activity (low, medium, high): is there a community support? Is the development team close to the community?
Platforms (Windows, Linux, OS X, etc.)
Development language(s)
Customizable theme/UI (yes/no, limitations)
Accessibility potential (name of the UI toolkit or any a11y related information)
Internationalization (i18n) system

When there is no info available for a criteria, the mention N/A is displayed.

List of clients

Name	License	Confidence	Community	Platforms	Languages	Custom UI	Accessibility	I18n	Notes
Alpine	Apache License v2	low	low (no ML)	Linux/Windows	C	?	?	?	maintained with patches, this project seems a bit hard to follow.
Claws Mail	GPL v3	high	high	Linux/Windows	C	No	GTK+ toolkit	gettext (PO files)	-
~~Gnus~~	-	-	-	-	-	-	-	-	Emacs extension
~~Mulberry~~	-	-	-	-	-	-	-	-	Last update 21-Feb-2007
~~Mutt~~	-	-	-	-	-	-	-	-	Require Cygwin etc on windows
~~SOGo~~	-	-	-	-	-	-	-	-	Groupware
Sylpheed	GPL	?	?	?	?	?	GTK+	Yes	Japanese Wiki
~~Zimbra Desktop~~	-	-	-	-	-	-	-	-	Commercial project
~~SeaMonkey~~	-	-	-	-	-	-	-	-	All-in-one Internet suite.
Thunderbird	MPL	high	high	Linux/OSX/Windows	C, C++, JavaScript, CSS, XUL, XBL	Yes?	Mozilla products are known to be accessible.	Yes	-