We wanted an overview of which domains our users were using in a certain country (Netherlands in this case). So, a simple Powershell snippet that counts all unique domains it encounters in the ProxyAddresses field of all users under a certain OU.

Note that if you have set contacts / forwarders, some domains that appear may not actually be accepted domains in your exchange organization.

#Author: Jos Lieben (OGD)
#Date: 13-06-2016
#Script help: www.liebensraum.nl
#Purpose: retrieve all unique domains in use under a specific OU and count them
#Requirements:
#active directory PS module
########

ipmo activedirectory

$users = get-aduser -Filter * -Properties * -SearchBase "OU=Netherlands,OU=Countries,DC=lieben,DC=nu" -SearchScope SubTree

$domains = @{}
foreach($user in $users){

$emails = $user.ProxyAddresses
foreach($email in $emails){
$domain = $email.Split("@")[1]
if($domain){
$domains[$domain] += 1
}
}

}

Write-Output $domains