MacMail won't let me send emails after I cleaned cookies. Will open incoming emails.

After I cleaned the cookies because things were slowing down, suddenly had trouble with MacMail. Comcast said not their fault. Fiddled with system preferences and got it back online. However, now it won't send out emails (will open new ones, though) and lists server as iCloud, which is NOT what I normally use—comcast.net instead. How do I get outgoing back online? (BTW, I'm NOT very techie, so speak in plain English, please!)