It has to do with the way EPiServer Mail collects statistics for opened mail.
The way this used to be handled, mail clients that do not load images and other externally referenced contents in mail would not count in the "opened mail".
In the current version of EPiServer Mail, 4.4 SP1, this has been changed so that the "opened mail" figure will take into account that anyone who has been registered to have clicked any links also should count as having opened the mail.
I sent a mailing a couple of months ago with about 400 recipients. The stats says "0 opened" but "74 clicks" and the form the mailing linked to has had alot of registrations. Anyone have any idea why EPiServer Mail thinks that zero mails had been opened even though they apperantly have?
Keep any answers simple, I'm not a developer. :)