Mail.dll doesn't download any HTML content from the remote servers.
It doesn't change how the images are referenced from within the HTML body.
The only situation in which Mail.dll changes that is when creating new email and loading HTML data from disk. Images located on disk are added to the email as visual attachments and HTML is modified to use cid references.
[Edit]
I've checked and it seems there is a bug indeed. We'll correct that and let you know.
[Edit2]
Corrected.