A companion of the MozillaFirefox browser, MozillaThunderbird is an OpenSource email client that comes with a RssViewer. GetItFirstFromHere if you want an alternative to MicrosoftOutlook or MicrosoftOutlookExpress. 

More comprehensive reviews have not been issued yet. See http://www.pcpro.co.uk/news/66786/thunderbird-10-email-client-bids-to-repeat-firefox-success.html for an initial report.

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'''Q:''' Can it be configured to use HotMail and Yahoo mail servers? 

'''A:''' Not out of the box, but the third party apps that slurp down hotmail and yahoo mailboxes could probably expose it as a POP feed.  Would take some hacking to do though, since Thunderbird doesn't implement the full range of MAPI that many of the windows versions of these adaptors use.

'''Q:''' Has it got any extension mechanism similar to FireFox XPI? I would be particularly interested in linking it to VbClassic applications.

'''A:''' Its extension mechanism ''is'' XPI (XPCOM actually, xpi is the file extension that means "XPCOM Interface").  There are a scant few extensions, and it might be instructive to grab some of them and tinker with them.  Be sure to share any results that you can -- documentation is otherwise so scant that aspiring hackers need the examples.

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See Also: MozillaBrowser