Mail under 10.9 sends emails sporatically

Hi,

My wife is running OS 10.9 on her 17 inch PowerBook. Occasionally when she tries to send an email, she gets a message "Cannot send message using server "XYZ". Other times using the same email address to send, the message goes through.

In looking at the data in the "Accounts" area everything looks like it should. Several times I have deleted the SEND information and set it up again. But the sporatic sending continues.

Any ideas of what might be going on and what I can do to fix the problem?

Thanks