[exim] greylisting issue

Hi,

I am using Exim 4.50 and greylisting for an email gateway. I receive an
email not in the database; Exim sends the 451; and the vast majority of
mail servers resend. It's the few mail servers that I need to receive
mail from that don't resend that I'm concerned about. For example,
Southwest Airlines does not always resend. This is a big problem. I'm
wondering what people on this list who use greylisting do about this.
Would it be better to send 450, for example? I've tried whitelisting,
but that's ex post and thus kind of useless.

Thanks,
Craig Jackson