avoiding being seen as spam

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.