Since this is new, the question will probably come up: how is the number of marketable leads on the new Lead Dashboard calculated?
(Image from this post: Winter 2016 Release
The answer is at the bottom of this article Lead Database Dashboard:
Marketable Leads: The number of all-time leads for the workspace, minus the following:
This very similar to the characteristics of leads blocked from mailing, with the exception of the leads with email suspended in the past 24 hours.