Below is a powershell script you can use to find users in Active Directory with a specific mail domain and export the specific domain to csv.
Edit both @domain.com with the specific domain you are searching for within the users active directory proxyaddresses attribute
$users = get-aduser -filter * -Properties Name,UserPrincipalName,ProxyAddresses| ? {$_.ProxyAddresses -like "*@domain.com"}
$csvarray=@()
foreach ($user in $users) {
$userobject= new-object PSObject # Create Array Row
$userObject | Add-Member -MemberType NoteProperty -name "Name" -Value $($user.Name)
$userObject | Add-Member -MemberType NoteProperty -name "UPN" -Value $($user.UserPrincipalName)
$i=1
foreach ($pa in ($($user.ProxyAddresses))) {
if ($pa -like "*@domain.com") {
$userObject | Add-Member -MemberType NoteProperty -name "Address$i" -Value $pa
$i++
}
}
$csvarray += $userobject
}
$csvarray | Export-CSV c:\temp\specificproxyaddressexport.csv -notypeinformation