Craig:
There's actually a way around it in a crunch...I've put a 5 minute window.
Tim:
That's really not a solution. While your server may say, come back in 5, you don't have any control over how, when, or if, the sender will actually retry.
Patrick:
Afaik greylisting uses an RFC compliant method. So if the sending mailserver does not resend after a while then it is broken and should be fixed. FWIW I have used greylisting for more than a year now and in all that time I have only once seen a mailserver not resend.
And for some, once is more than enough. I don't use any spam filters, as one false positive is more than enough, and I've seen many more than one.
I'm not sure that a server really *has* to resend, but it still leaves you with a problem: You can't reconfigure someone else's server. And, you mightn't even be able to mail them to discuss it.
The point I was making, at the start, is to go into it with your eyes open. Be aware that you may lose mail, important mail, permanently.