Find user without mailbox

I dont have access to AD right now, but i think you could do something like this. ! in front of $user.msExchMailboxGuid means that if the variable is null(empty), it should create a mailbox, if its already have a mailbox, a message stating this is dispalyed. Hope this works, or sett you on the right track.

$Users = Get-ADGroupMember -Identity "S_Enable_Exch2010User" | Get-ADUser -filter {msExchMailboxGuid -like "*"}

foreach ($user in $users) {

if(!$user.msExchMailboxGuid) {
    enable mailbox command
}
else {
    $user.firstname+"already have a email account."
}
}