Bulk Mailbox creation by using Powershell

Run following command in Powershell Exchange Management Console

$password = ConvertTo-Securestring "P@ssw0rd1" -asplaintext -force
$i = 1
While ($i -le 10)
{
New-Mailbox -Password $password -Name "User$i" -UserPrincipalName user$1@contoso1.com -OrganizationalUnit "contoso1.com/Users" -Database MDB01
$i = $i + 1
}