Basically Greylisting is another weapon against spam.
When a request comes in to send mail to the smtp server, the request is denied with the message "Greylisting in action, please come back in 00:31:00".
My server tried to send a message at 11.11 am
Router: Message xxxxxxxx NOT transferred via SMTP to xxxx.com for xxxxx@xxxx.com 451 4.7.1 Greylisting in action, please come back in 00:31:00
The mail went into database mail.box on my server, as if the server had not replied.
My server tried dutifully to send it again at 11.18 am (BTW not 31 mins later) and the email was taken.
So the 31 minutes was not important for this implementation. You might need ot check your try times and try interval to handle this.
I then sent another message to see if I was now whitelisted, or if the new email would also be "greylisted".
Disappointingly it was greylisted again, ie so the email will be delayed again, so greylisting doesn't learn.
So Lotus Notes mail router will handle sending mail to a server that is using greylisting.
Why does Greylisting stop spam? How does Greylisting work?
It works because spammers do not usually use a mail queue, if it is rejected they move on to another address.
ie the spammers smtp server will not try again later.
Spammers will adapt eventually (probably quite quickly) , but they may decide not to bother.
Is there a Lotus Notes addon for greylisting?
I have not heard of one.
Greylisting Advantages
Will reduce spam for a while, which is a big advantage.
Works with most email systems for receiving out of the box.
Could be integrated with further whitelisting or blacklisting to fine tune.
Greylisting Disadvantages
Email is delayed by 15 to 30 minutes. I do not think this would be acceptable to some people.
Greylisting servers do not all learn, so subsequent emails are not fasttracked through.
Partial Whitelisting would solve this problem, but this adds admin time, may be it could be automated.
Links on Greylisting
http://www.greylisting.org
http://en.wikipedia.org/wiki/Greylisting