It seems you are confusing some things.
Sender may include tracking image into emails HTML.
It makes no sense to do that on the receiving side:
First, IMAP generally can't modify existing emails.
Second, receiving application knows when user downloaded and displayed specific email.
Maybe you need to describe your scenario in detail (e.g. how your users read emails, what is the purpose of the application you are developing) so we can understand it better.
Mail.dll only downloads emails from the server and marks them as read.
It doesn't hit your IMG src address.
To trigger this, you need to use IMail.Html property, extract IMG tag address and hit it with some HTTP client.