Can't send e-mail, but can receive

Dear All,

I'm working on a Mac Pro with Lion 10.7.5. I'm able to connect to the internet and receive e-mail through Mac Mail, but for several days have not been able to send mail.

This has been an intermittent problem with AT&T high speed DSL, but has never lasted this long. The error message says incorrect password and login, but I know they are correct because I'm receiving mail, just can't send. AT&T techs seemed to solve the problem this morning, but my send capabilities lasted for one email and now will not work again.

Is this a problem with Lion and Mac Mail or AT&T. They said something about the firewall being a problem.

Thanks.