On Wednesday 15 October 2008, Tonnerre Lombard wrote: [..]
Not very problematic for the mail server but of course the PHP script does _not_ attempt redelivery. And your users go to gmail, because there they get the mail. Not sure that's desirable for you.
This whole discussion is pointless. Greylisting is a religion. The believers worship it, the others damn it.
The realy important point is: Greylisting is a just using a mechanism that should get going when something is goes wrong accepting a message. This mechanics of retransmitting should not only take action with greylisting involved but (and that is the important point) when there appears a real technical problem. And that is something a customer with his little online shop will show open ears to you explaining him why to change his mailer script.
have fun
Michi