some mails bounce with 'Name or service not known'

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some mails bounce with 'Name or service not known'
\ Sven Putze (12 Jan 2008)
. \ (Wietse Venema) (12 Jan 2008)

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Subject:	some mails bounce with 'Name or service not known'
Group:	Postfix-users
From:	Sven Putze
Date:	12 Jan 2008

Every now and then Postfix doesn't deliver mail (it's bounced directly,
not deferred) and the reason is logged with

<account>: [dom.ain]: Name or service not known

But the target domain "dom.ain" is an existing one (and can be
successfully "digged". If the sender tries again Postfix can find
"dom.ain" and delivers the mail (as most of the time to this domain).

First question: is direct bouncing a wanted behaviour and if so, is
there a chance to get those mails in the deferred queue instead?

Second question: does Postfix have problems with DNS lookups when
spambots are causing stress?

Short info about the system:

Debian Woody (no chance for an update, I would if the customer would let me)
Postfix 2.1.5
local caching nameserver to speed up RBL lookups (the problem also
occurs with direct addressing of the computer centers dns servers)

Need any special parts of config files?

Thanks for help!
--
Sven