Anyone who has migrated a legacy ASP application to Win2003/IIS6 is familiar with CDONTS and CDOSys. Code that sends email via CDONTS stops working in W2k3/IIS6 because there is no CDONTS.dll!
Code must be migrated from using CDONTS.NewMail to CDOSys.Message. Required changes are fairly straightforward.
Here's a subtle difference: your ASP application under CDONTS will not visibly fail if you provide an empty string "" for the email address. Rather, the email message will fall into your ISP's bad mail bin. CDOSys, on the other hand, will throw an error, and the application will quit on the end user if the error is unhandled.
Here are proof of concept scripts to show you the variance. You can run these from a command prompt:
wscript.echo "creating cdonts object!"
dim mailObj : set mailObj = CreateObject ("CDONTS.NewMail")
mailObj.From = "TestAddress@blogger.com"
mailObj.To = ""
mailObj.Subject = "hallo"
mailObj.Body = "hello there"
mailObj.BodyFormat = 1
mailObj.MailFormat = 0
wscript.echo "mail sent, supposedly"
set mailObj = Nothing
dim objCDOMailer : Set objCDOMailer = CreateObject("CDO.Message")
objCDOMailer.To = ""
objCDOMailer.From = "TestAddress@blogger.com"
objCDOMailer.Subject = "hallo"
objCDOMailer.TextBody = "hello there"
wscript.echo "Message sent successfully!"