I have gone through the bulk mailbox creation for all of the users in AD. The steps included were creation of a CSV file with user atributes and run a PS script which processes the csv file.

1. Crate a CSV with the following fields for all AD user objects: Lastname,Firstname,Name,UserPrincipalName,Password

2. Make the script and save as ps1 and run the script :

Import-CSV C:users.csv | ForEach-Object -Process {New-Mailbox -Name $_.Name -FirstName $_.FirstName -LastName $_.LastName -OrganizationalUnit $_.OU -UserPrincipalName $_.UPN -Alias $_.alias -Database “Name of database” -Password $Password}