I’ve considered posting a question on the forum lately to ask if Mail.app has been slow to load for folks lately because it sure has for me! Then, today I see this and (1) it can’t be right? (2) what the heck? (3) would this explain slow loading?
Something is wrong. I surely can’t have that many messages. Any ideas?
I don’t have an answer but what does it show if you look at mail on iCloud?
Looks like you have some catching up to do. 
From time to time it appears that my Apple mail well sometimes decide that it wants to re-download previous IMAP email for no particular reason. Kite slowly during this time. If you open the activity window you can see what is happening.
Although it is odd, I have learned to just let it do it’s thing and then it will be OK for a while.
Haha, that is one less than the maximum value of a 64-bit binary, i.e., it’s 111111111111111111111111111111111111111111111111111111111111110
Did you read one? 
Thats a bigger number than there are grains of sand on the beach
I think that is a novemdecillion.
Perhaps this is a clue? Kind of like when a file’s date is set to 00:00:00 UTC on 1 January 1970.
Are all the emails from the same person or organisation? I once had abut forty identical messages from the same company.
It’s nine quintillion two hundred twenty-three quadrillion three hundred seventy-two trillion thirty-six billion eight hundred fifty-four million seven hundred seventy-five thousand eight hundred and six.
Have you considered the “Inbox Zero” approach 
That’s properly bonkers.
Wow!
Do any individual mailboxes seem to show a large unread count or similar problems? If so, I’d try rebuilding those mailboxes. It’s the Mailbox > Rebuild menu item.
If that doesn’t fit, you may need to force Mail to reindex. I had to do this last year because of Mail slowness. Here’s a reliable explanation of how to do it:
That is one big number, that looks like my tax bill!
My GTD Trainer talked about E-Mail processing today, and he said, that processing 20 Mail, within 10 Minutes, would be fast.
So…
So, that’s
a hundred Mails per hour,
800 Mails in a 8Hour-Shift.
2400, if you work 24/7
16.800 per Week
876.000 per year.
So you would be done in a little bit over 10.528.963.512.391 years!
But you have to consider a little lack of sleep on that in between, so… 
I want to see that on an IOS app red notification.
Just to put that it words, you have over 9 quintillion messages to read. Better get moving. If you read one per second, it will take you roughly 2.922770246x10^11 years to read. Maybe this Mac Power User community will be kind enough to provide a food train for you so you don’t have to take breaks from your computer. Please post your dietary preferences.
I missed your calculation when I drafted mine! Either way, @beck has a lot of work on her hands.
I’m pretty sure it caps out at 31,000.
Well, I tried several things (rebuilding, reindexing, and Clean My Mac mail speed up things) that didn’t work, but this process has you rename the Mail folder and the mail-related preferences files so that Mail sets them up again and now it’s super zippy. I had to add my accounts again, which was an hours-long to do, but that fixed it.
Showed this to my wife who often has 3,000 to 4,000 unread messages in Mail. Her comment, “I’m glad it’s not me.” I’m in the inbox zero camp and sometimes nag on her about cleaning up her mail.
