The checks are done at RCPT TO time because some MTAs treat 5xx errors at times other than RCPT TO as temporary errors.
These checks are cheap because we only look at envelope data, not at message contents.
ASBL (automated sender blacklist): e.g. If we receive 3 junk mails from firstname.lastname@example.org in 6 hours, we blacklist that address for 24 hours. Experimental
The Yahoo! Groups block stops rogue attempts at subscribing python.org mailing lists to Yahoo lists.