Or put more simply, how many John Does are there in the world, and how many of them might like firstname.lastname@example.org?
You have to consider the growth of email and the increasing number of email users over the decades to understand how high the odds are against anyone scoring email@example.com as their unique email address.
The first email message across a network -- QWERTYUIOP as a test -- was sent in 1971.
In the 1980s most email use was by government and military personnel and students and professors in academic environments, but email use exploded when Tim Berners-Lee created the World Wide Web in 1991
By 2012 there were more than 3 billion email accounts across the world, sending and receiving around 294 billion emails a day.
So what are the odds John Doe can be successful at securing his long-desired johndoe@ address?
Well, based on the number of global email users around the world, the odds of John buying a winning lottery ticket are better.
The situation isn't quite that bad, of course; if John fails to get firstname.lastname@example.org -- and doesn't want to be email@example.com, or johndoe987654321@ for that matter -- he might try snagging johndoe@ at another email service like gmail.com or outlook.com or aol.com.
But that only needs three John Does to get there before he does and again he's out of luck.
He can improve the odds a bit more if he's willing, as millions of people are, to accept some variation of his name, like jdoe or doejohn or johndoe1962 (birthdate).
But that's like having to settle for a "not quite perfect" vanity license plate for your car because someone else got to the motor vehicle department first with the same idea.
So the chances for our John Doe to have the world's only johndoe@yahoo address remain somewhere beyond slim.
And he has to accept that his name is itself part of the problem; someone named Wilberforce Heppelthwaite is likely to face much less competition.
So the John Does of the world will have to accept that even given Yahoo!'s new policy, they need to start considering what they would accept as an alternate choice.
Might even have to give up on the personal name idea completely, and go for firstname.lastname@example.org.
Oh, sorry. Already taken.