Setting up mail on mac

[twilitegxa]

I have had my mail set up on this computer and it worked fine, then all of a sudden one day the outgoing mail stopped working and now I can't figure out how to fix it. All of my incoming mail appears to be working just fine and I am unaware of any changes I've made that could have caused this problem to happen. I have several accounts: two with comcast, one with aol, and one with mac. I don't understand why it just all of a sudden decided to stop letting me send my e-mail but will still let me receive it. Any suggestions?