$searchbase="OU=Users,DC=Domain,DC=DE"

$users=get-aduser -SearchBase $searchbase -filter * -Properties emailaddress

foreach ($user in $users)
    {
    if ($($user.emailaddress) -ne $null)
        {
        write-host $user.SamAccountName $user.EmailAddress $user.EmailAddress.ToLower()
        set-aduser $user.SamAccountName -EmailAddress $user.EmailAddress.ToLower() -confirm:$false
        }
    }