Milter-greylist

> Actually, these power users should normally have installed some  
> regular MTA,
> featuring the ability of relaying mail from the outside.
>
> That mean they should commonly have port 25 open on their computers,
> available for SMTP connections.
>
> Even if that MTA is not available for open-relaying, could we just  
> check if
> something is open on port 25 (with a consistent greeting message)  
> on the
> sender's IP address, without actually trying to send any message  
> through
> them, and decide to lighten back the greylisting on that basis ?

oh, this sounds like what Verizon does.

It's amazing what traffic I get when I had verizon blocked, then  
tried to send them an email.
I swear, I must of had 2,000 bounce messages in just a couple of hours.

Bill