421 Unexpected failure, please try later

I have been running NIS 2010 on my Windows XP 32 bit PC for a long time, but all of a sudden it throws up a dialog box whenever I try to send out an email. The message in the dialog box is:

 

"421 Unexpected failure, please try later" followed by the details of an email message.

 

The email in question is just a simple text message without any attachments, so I cannot see what the problem could be. Any ideas?

HI tonymarston

 

Welcom e to the Norton Community Forums

 

The message you are getting is from your SMTP mail server. You could check the following items

 

Make sure your username in your e-mail client is user@domain and not user+domain
Make sure your account is not over the quota limit
Make sure the individual e-mail account is not over its quota limit
Make sure you're not attempting to send very large attachments via e-mail (Anything over 10MB and its best to upload it rather than send via e-mail, it wasn't designed for large attachment)
Make sure more than one person isn't trying to access the same e-mail account while you're sending / receiving
Make sure your have "My server requires authentication when sending e-mail" (Exact wording may vary) enabled on your e-mail client, you need to authenticate before sending