Just FYI, I've had the same problem for months now. I have been accessing my hotmail via the web, but decided to try the OS X Mail app. I could get emails into the account with no problem but couldn't send. I read all the blog and support entries and have tried everything they recommended, like changing my computer name to one word, without success. Then last night, it just so happened that my broadband provider had to change out my wireless modem. As soon as he plugged in the new one, some emails that had been in my outbasket for a month went through instantly and all of a sudden my Mail, both incoming and outgoing now work. So I think people need to look at their router settings for the a solution to the problem.