do you get an error message when sending mails? are you using a client mail or the webmail? if client mail, ensure that you verify your ability of sending/receiving mails over the web first. that way, you could tell if the server is down or not.
Hi jvfogel there are so many factors for this type of situation. more often if you cant send emails both form att yahoo and outlook then this could be because of your virus protection.try to disable it for a short while then try sending your emails again.if the same thing happens then try to call att technical support i know they are open everyday even during night time.alot of people from that department is very helpful