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.
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.
See Also: MozillaBrowser