Suddenly cannot send mail but can receive

I am having trouble with Mail on our MacBook Air. Mail was working normally when I first started using it this morning. I sent and received a number of messages. Then, in the late morning, I found that I could not send email any longer. However, I continue to receive email. I've tried a number of times and having been getting the same message -

We have an iMac where Mail has the same account settings. We are receiving and sending mail from that device just fine.

Have tried everything I could think of including the last ditch Shut Down and Start-up. But nothing seems to work.

Look forward to a response.