cannot send email

All of a sudden I cannot send email.  Am receiving the following message:

Sending of the message failed.
An error occurred while sending mail: Outgoing server (SMTP) error. The server responded:  resomta-a2p-647345.sys.comcast.net resomta-a2p-647345.sys.comcast.net Excessive failed authentication, ESMTP server temporarily not available.