Error during send of email

I have a VB.Net program that sends an email to someone with status info. For some reason, I get an error every so often. This error is very speratic and I can't seem to track it down. The message is..

The server rejected one or more recipient
addresses. The server response was: 550 client.

Has anyone else run into this problem before? It does not happen all the time. I can try to resend the email (in the applicaiton) and it usually works fine the second time I try with no changes. Any ideas?

Thanks

I thought I read that 550 was thrown when the email server does not recongize the ReplyTo address (used to prevent spam / hackers / viruses??).

It might also be due to the receipients email server or their account (full, temporarily suspended, etc).

Don't have much experience with this, so I am just trying to provide some suggestions.

Anyone else have ideas?

I found a post that says something about configuring SMTP and the "relay" button. That is, Relay restrictions. The post says to add the server's IP address under the Relay restrictions, to allow. I will give this a try and see what happens.

Thanks!