don't just include the link, it has to be wrapped inside an image tag like
<img src="http://www.anywhere.com/viewemails.asp?ID=4">
viewmails.asp has to write the image and update the db. Note that many email and AV programs can prevent scripts from being run from within emails.
An alternative is to to just use the direct link to the image within the email with a query string appended to it like...
<img src=http://www.anywhere.com/myimage.gif?in=xxx>
In this situation, you'd write a script to scan your log files and update the db appropriately.
Neither of these two will return you anything for Outlook 2003 email clients where images are turned off and not downloaded to the client by default unless the user specifically requests it.
Edited by dpyers