Milter-greylist

Hi,

Some legitimate emails get greylisted for 90 hours and more, and I'm trying to
understand why.

What cases would cause such very very long greylisting?

I frequently (a few times a day) also see greylisting of emails for 7 hrs and
a little more, and looking at logs I can't understand why.

So I'm also wondering, what analysis / trouble-shooting should I be performing
to determine why such lengthy delays occur? and what can I do to "fix" such
issues other than whitelisting the sending domain.

Thanks.

Michael.