When does smtpd_reject_unlisted_recipient kick in?

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When does smtpd_reject_unlisted_recipient kick in?
\ Stefan Foerster (22 Sep 2007)
. \ (Wietse Venema) (22 Sep 2007)
. . \ Stefan Förster (22 Sep 2007)
. . . \ Justin Piszcz (22 Sep 2007)
. . . \ (Wietse Venema) (22 Sep 2007)

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Subject:	When does smtpd_reject_unlisted_recipient kick in?
Group:	Postfix-users
From:	Stefan Foerster
Date:	22 Sep 2007

Hello there,

the documentation about smtpd_reject_unlisted_recipient states:

,----[ man 5 postconf ]
| Request that the Postfix SMTP server rejects mail for unknown
| recipient addresses, even when no explicit reject_unlisted_recipient
| access restriction is specified.
`----

postconf -d confirms that this is set by default, and so far, it is
doing exactly what I expect.

What I'd like to know is: When is this setting evaluated? At the end
of the restrcitions listed in smtpd_recipient_restrictions? At the
beginning? Does it change the behaviour of existing restrictions which
are valid in the context of smtpd_recipient_restrictions?

If my smtpd_recipient_restrictions contain some very "cheap" lookups
(e.g. in a hashed map) which would already cause a lot of mail to be
rejected, could setting smtpd_reject_unlisted_recipient to no and
explicitely state an reject_unlisted_recipient _after_ those cheap
lookups improve the overall throughput?

If this question is already answered in the documentation and I simply
didn't search hard enough, please just tell me where to look for an
answer.

Cheers
Stefan