System.web.mail: Delivery

Hello!

I have a .NET service that sends out about 400 e-mails every month. The
e-mail is plain text, with a hyperlink. Some e-mails appear to be bouncing,
though they actually are delivered!

I am using VS.NET 2003 and system.web.mail to send the e-mails.
This is in short the message I get back:
This is an automatically generated Delivery Status Notification.

Delivery to the following recipients failed.

xxx@xx.xom

Anyone has an idea of the cause? Bug in system.web.mail?

Regards

Clarkie