I would first make sure no settings have been changed in Mail>Preferences. If nothing has been changed most of the time it is still a settings issue, I would check your outgoing server port for each account that isn't sending(you do this by going into Mail>Preferences then click on the accounts tab, then the account information tab, and there should be a server settings button on the bottom of the page) by default the port is set to 25, you can change that number to 587 and see if mail sends after that.
If you are still having trouble here is a general article for being unable to send email:
http://docs.info.apple.com/article.html?path=Mail/2.0/en/ml1041.html
And also any error messages you are getting would be helpful as well.