DNS-Prefetch could be a security leak in emails?

As you may know that, email clients and webmails usually disable auto-loading of external images (and others) to prevent spammer getting confirmation of the reading activity.

There are usually an external image with a unique URL to a image that spammers will know which emails are read, which addresses are active, when the image being loaded.

DNS-Prefetch won't  load any URL, and it won't submit any request to the web server. However, it will submit request to DNS server.

If spammer improve the method, by adding a link to a URL with a sub-domain which is unique? (possible for wildcard DNS)

Then a request (confirmation) might send to the spammer when you opening the email, without your permission.

Lucky, it seem that the DNS-prefetching is disabled by default in HTTPS.

Ref: http://codemonkeyism.com/google/