Send Mail has stopped

Today my MacBook Pro Mail App is giving a message that it can't send mail and the email is stuck in the Outbox. I have tried the various suggestions in the error. All fail. When I click on Edit SMTP List there is no SMTP server listed. Am I supposed to have one? I don't recall ever setting one up. I thought the mail app account setup automatically sorted this?

Also we have two users on the laptop. This error occurs for both users.