Trouble sending a link or page

Suddenly I am unable to send a link or a page using mail. I have used the connection doctor. I am connected, but the box says: cannot send message using the server smtp(followed by my user name). I don't think I have changed anything. I know all the ways to send links on my MacBook Air and have never had a problem before. I have no problems sending or receiving mail. I have version 10.10.5 Yosemite. I have tried sending a link and a page with the same result. I have talked to Comcast and they could not help me.