Symantec Email Proxy error: 421 Resources Temporarily Unavailable

I am using Symantec Endpoint Protection version 11.0.5002.333 on three XP+SP3 destops.

I keep getting the following "Symantec Email Proxy" error message after sending email:

"Your email message was unable to be sent because your mail server rejected the message:
421 Resources temporarily unavailable - Please try again later"

There is a link on the error message popup that always points to a “Upgrade your Norton product” page.

I have been getting this error periodically for over a year. It does not happen every day, but has occurred several times on the same day. I have to resend the email, sometimes several times.

What is the cause and, more importantly, the solution.