How to get GUID all of user in OU

I have use exchang svr 2013 .

How to get GUID of user all in OU ?

currently i use scriptr exchange power shell

$username = "username"
$guid = (Get-Mailbox $username).ExchangeGUID
$upn = (Get-User $username).UserPrincipalName
$upnsuffix = $upn.Split("@")[1] 
$ServerName = "$guid@$upnsuffix"
write-host $ServerName