I set up greylisting on our MTA and eventually had to remove it. While I was okay with waiting a while to receive a message other people were incredibly frustr

I set up greylisting on our MTA and eventually had to remove it. While I was okay with waiting a while to receive a message other people were incredibly frustrated with it. It was particularly troublesome when a service would send them a code in email, it didn't arrive, and then they would ask for another one. The first code would finally arrive, but it was marked as invalid because the second code that they had asked for was now the valid one. It just wasn't worth the trouble and headaches.