A quick explanation: “Greytrapping as implemented in spamd puts offenders in a temporary blacklist, dubbed spamd-greytrap, for 24 hours. Twenty-four hours is short enough to not cause serious disruption of legitimate traffic, since real SMTP implementations will keep trying to deliver for a few days at least.”

How does it work? (From another article linked in this post)..

“We already know that spam senders rarely use a fully compliant SMTP implementation to send their messages. That's why greylisting works. Also, as we noted earlier, not only do spammers send large numbers of messages, they rarely check that the addresses they feed to their hijacked machines are actually deliverable. Combine these facts, and you see that if a greylisted machine tries to send a message to an invalid address in your domain, there is a significant probability that the message is a spam, or for that matter, malware.”

OT: This site has another recent article I found even more interesting-

A Major Mail Provider Demonstrate They Likely Do Not Understand Mail At All https://nxdomain.no/~peter/they_do_not_understand_mail_at_al...

Gotta love the email self-hoster holdouts, mad props and respect.