error message - server rejecting recipient

I am getting this error message below and not sure what do?

I regularly email up to 150/200 customers with updates and never come across
this problem. Sometimes I get email bouncing back as email address no longer
know but the email is still sent to the rest of the addresses.

HELP I am really stuck.

Ken



Error: "Your email message was unable to be sent because your mail server
rejected the recipient." (1003,10)

Situation:
After installing Norton AntiVirus, when you send email you see the error
message "Your email message was unable to be sent because your mail server
rejected the recipient." In the lower-left corner of the error display
alert, you see the following set of numbers:
1003,10

Solution:
Causes of the error
This error can occur for any of the following reasons:

The email that you are sending has an incorrect email address.
The email address that you are sending the email to is no longer available.
You have been blocked from sending email to that email address by the
Internet Service Provider (ISP) of the recipient.

What to do
To solve the problem, make sure that the email address of the recipient is
entered correctly. Also ensure that the person whom you are emailing is
still using that account.

If the error continues, contact your ISP for further assistance.