An error occurred while sending this message using the SMTP server “(null)”

An error occurred while sending this message using the SMTP server “(null)”

Hi, I have not been able to send emails via Mac Mail for two weeks now. I have two different computers that access the same IMAP account (MacBook Air, and iMac) - the problem is identical on both computers and started at exactly the same time.

The error message is:

"  Sending the message content to the server failed. The server response was: An error occurred while sending this message using the SMTP server “(null)”.  Select a different outgoing mail server from the list below or click Try Later to leave the message in your Outbox until it can be sent.  "

I thought something was wrong with the server based on other user complaints from 14-12 days ago, but my error messages still have yet to be resolved. I have to use the web-based mail exclusively for sending mail. I can indeed receive email on Mac Mail, and all the SMTP server settings appear to be correct - I have not tinkered them at all for many months if not years. Mac Mail's 'Connection Doctor' for both IMAP and SMTP settings seem to be fine (green light with message : "Connection and login to server succeeded.")

Please help?