Something that I realized is that the accounts are not in the same sever. I tested with 3 accounts and only mine (the updated one) failed
Yes, you need to be admin (Office 365 admin) to check this info
You need to open powershell (as admin -right click run as admin-) and run this command
Set-ExecutionPolicy RemoteSigned
Install-Module -Name ExchangeOnlineManagement
after the install and several Yes you can connect to your tenant using
Connect-ExchangeOnline -UserPrincipalName ******@yourdomain.com
After that you can check every mailbox using
Get-Mailbox ******@yourdomain.com | fl *version
It should return something like this
AdminDisplayVersion : Version 15.20 (Build 7409.32) <--- this version is the one that seems to be working
ExchangeVersion : 0.20 (15.0.0.0)