I don’t know about you, but I still create my users using an old VBS script. It creates the users fine but the Exchange Server is 2007 so it creates a legacy-mailbox type which can’t be used from OWA.
Easy fix, in PowerShell, search and apply mandatory properties to fix them. You can take this on the end of a VBS script if you call it from a PowerShell prompt:
Get-Mailbox -server SERVERNAME -RecipientTypeDetails LegacyMailbox | set-mailbox -applymandatoryproperties