Greylisting is 3V1L

As a concept you tend to either like it, or hate it.
People expect email to be many things - they expect it to be instant, they expect it to be endless in capacity, they expect it to be able to shift endless quantities of data.
On the whole this is why there are instant messaging protocols. There are long term storage solutions. There is FTP and SCP.
Getting back to my point - Greylisting ... the concept of delivering a 451 or something and issuing a "not right now mate, try again in a bit" as a temporary failure - expecting the sending server to come back again in a few minutes.
If it does - then you make a note of the sender, recipient, and the server involved, and do not slow up mail from that triple again.
Sure it is a pain for the occasional password resets - or mails from people that you have not heard from in a while.... but if you are sitting there reacting to emails arriving in a Pavlovian style then that itself says a lot for how you manage your time.
So hard stats.
Here are some stats from a reasonably busy host here - for a days worth of email.
Of 685938 items that were initially greylisted:
- 9722 ( 1.4%) became whitelisted
- 676216 ( 98.6%) expired from the greylist


That is 98.6% of all mails failed to come back.

That is 98% less DNS lookups to RBL's.

That is 98% less processor and IO involved in processing emails and scanning them for nastyness.

That is 98% less rules and heuristic scanning of mail.

The real stuff gets through, the real stuff is getting more resources.

It holds up what is the accepted industry stat - for every one email that you get - NINE have been rejected.

I will try to be less grumpy about the occasional spam I do get amidst the constant stream of emails.


Anyway - the point - a stat worth sharing. Greylisting is not evil.

It is at worst an occasional inconvenience.

*Man up. Deal with it.

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