If you have a quite a large Exchange / Active Directory environment then sometimes when you want to give specific users mailboxes and you use the Exchange 2016 / 2013 EAC to create users you see the following notification and cannot find the users you want to give a mailbox.
This basically means there are too many results to show, the downside of this is with the gui if you cannot see the users you want to mailbox enable then you cannot enable them and will have to use powershell. To do so do the following.
get-user -OrganizationalUnit "THEOU" | enable-mailbox -database "Mailbox Database X"
Where “THEOU” is the OU you want to enable and “Mailbox database X” is the friendly name of the Mailbox Store. THis will create a mailbox for every user in that OU on that particular mail store.