Assuming that you use a template system for emails:

email.tpl
Code:
[website_logo]<br /><br />

Dear [user_name],<br /><br />

This is an email sent via CDO from my website [website_name].<br /><br />

[tracking_bug] <!-- this is where the tracking image is inserted --><br /><br />

Regards,<br /><br />

[email_sender]
replace [tracking_bug] it like so:

Code:
strMsgTemplate = Replace(strMsgTemplate, "[tracking_bug]", "<img height=1 src=""http://www.mywebsite.com.au/asppage.asp?a=" & intUserID & "&b="&intEmailID&""" width=1>")
Pretty simple stuff... now the back end will be something like:

asppage.asp
Code:
<%option explicit%>
<!--#include virtual="/inc/functions.asp"-->
<%
Dim strSQL
Dim intuser_id		:	intuser_id		= Numeric(Request("a"))
Dim intnewsletter	:	intnewsletter	= Numeric(Request("b"))

strSQL = "Insert Into email_track (user_id,newsletter) Values("&intuser_id&","&intnewsletter&")"

OpenDB()
adoConn.Execute(strSQL)
CloseDB()

Response.Redirect("/images/transparent.gif")
%>
This needs to be sent as a HTML email, email clients who block images or convert to plain text will not be recorded with the tracker.

If anyone knows how to track non-HTML/Image suppressed emails please let me know (dont think it is possible)