Mark - my recent experience with this was that Rackspace was blacklisting '
notify@quickbase.com' and stopping emails from getting through. I switched it to "a User" and it was sending out as "xxxx on behalf of
notify@quickbase.com". That still failed to get through. I then switched it to "application owner" which was not sent out as "on behalf of"... which solved our problem. I wonder, in your case, whether creating a robot user email account on the Client's server and switching the Owner of the account to that robot user and then choosing that as the sender might help. I am sure someone else has had the same issue and might have solution options.