Thunderbird? Forget it.
If I try to send mail to Hotmail users from Thunderbird, they simply do not receive it, nor do I receive any error. If I resend the same mail from Outlook it reaches the users just fine.
If you're a Hotmail user you should understand that you are losing legitimate email thanks to Microsoft's dubious, heavy-handed, enforced 'anti-spam' measures.
I've created brand new test Hotmail accounts using random localparts - not guessable and completely unlikely to have ever been used before (eg s2f8du8xbgaaz@...). Some of them have started receiving spam within one minute of signing up (literally read the 1 new welcome email and deleted it, inbox refreshed, still with 1 new unread email now for pills). There was no malware or trojans or cross-scripting going on in these tests.
Somehow, new accounts were being slurped off in places and put straight onto spam lists. On the other hand many of the test accounts never received a single piece of spam (my gut feeling was load-balancing and a publicly accessible weakness on some but not all of the relevant systems)
As ever, Microsoft were not interested and didn't respond or else gave useless form responses. Their slapdash, heavy-handed approach in dealing with spam, and again in dealing with viruses via MSN, is an allegory for their sloppy attitude towards their users - mere fodder to drive the real revenue-making systems, treated with contempt and kept in the dark, or else kept dumb with marketing spin.
It's not just a case of Hotmail being technically very poor as an email system these days - Microsoft frankly don't deserve that user base any more. They certainly haven't done anything great with Hotmail, and compared to other offerings it's piss-poor. Protecting us from evil spam by throwing half the email away isn't what we expect by "driving innovation".