I can receive mail from Earthlink but can not send from my MacBook.

I can receive mail from Earthlink but can not send without the mail being returned. I work on a Macbook 10.7.5. 4 GB Mid 2009. We have contacted Earthlink, they say they have corrected the problem, but it's not corrected. I would appreciate any help, this is wasting so much time trying to send business documents back and forth.