You've probably heard about this one before, but not necessarily for email campaigns.
This is because the Cost Per Thousand term is actually an industry standard measure for ad impressions.
In our case, email has a relatively low CPM, if you compare it to other marketing channels.
If this sounds a bit tricky to you, then you should know that the CPM is actually just a simple term
that shows the price of 1,000 advertisement impressions on one webpage and a very common methodology to price web ads.