Email Contacts receiving repeat message generated by TMDA

Hi,

Recently several contacts that are listed in my address book have been complaining that they repeatedly get asked to confirm their message.

I do not know what causes this message.  I would like to stop it from happening.  My email is on my own domain, theplanetearth.us.  I have Norton antivirus and qurb spam blocker. 
I don't know how to tell what is generating the message.

Below is the text of a recent message from an address book contact.  

Please tell me how I can stop this message from being sent to those in my contact list.

Thanks,
[email protected]

Message received today, July 24, 2006:

I keep getting this message every time I send you something. 
I don't know what's going on.  It says I shouldn't get it more than once but I am. 
----- Original Message ----- 
From: theplanetearth.us 
To: [email protected] 
Sent: Monday, July 24, 2006 11:13 AM
Subject: Please confirm your message


This message was created automatically by mail delivery software (TMDA).

Your message attached below is being held because the address
<[email protected]> has not been verified.

To release your message for delivery, please send an empty message
to the following address, or use your mailer's "Reply" feature.

   [email protected]

This confirmation verifies that your message is legitimate and not
junk-mail. You should only have to confirm your address once.

If you do not respond to this confirmation request within 14 days,
your message will not be delivered.