[Exim] Failure modes for NIS+ lookups?

Admittedly I don't understand NIS+ as well as I should, but is there any
way to differentiate a lookup failure of "user doesn't exist" vs "I can't
lookup this user right now because the NIS+ server is unavailable"

I'd like to bounce on the former and freeze or retry on the latter.

For example I have the following vacation message autoresponder
transport/directors in my config.

vacation:
  driver = smartuser
  condition = "${if or {{match {$h_precedence:} {(?i)junk|bulk|list}} \
                        {eq {$sender_address} {}} \
                        {def:h_List-Owner:}} {no} {yes}}"
  require_files = ${local_part}:${lookup nisplus{[name="${quote_nisplus:$local_part}"],passwd.org_dir:home}{$value}fail}/vacation.txt
  transport = vacation_delivery
  user = ${lookup nisplus{[name="${quote_nisplus:$local_part}"],passwd.org_dir:name}{$value}fail}
  unseen
  no_expn
  no_verify

vacation_delivery:
driver = autoreply
subject = "${if def:h_Subject: {Re: $h_Subject:} {I am on vacation}}"
file = ${lookup nisplus{[name="${quote_nisplus:$local_part}"],passwd.org_dir:home}{$value}fail}/vacation.txt
log = ${lookup nisplus{[name="${quote_nisplus:$local_part}"],passwd.org_dir:home}{$value}fail}/vacation.log
once = /var/mail/${local_part}/Maildir/vacation.once
once_repeat = 7d
to = $reply_address
from = $local_part

And if mail comes in for an unknown user the following paniclog entry is
generated.

2000-11-27 12:36:15 140RwM-0007PT-00 expansion of ${local_part}:${lookup nisplus {[name="${quote_nisplus:$local_part}"],passwd.org_dir:home}{$value}fail}/vacation.txt failed: lookup failed and "fail" requested: NIS+ error accessing entry [name="username"],passwd.org_dir: Not found

Where this is particularly annoying is if there are several recipients on
one message and only one of them no longer exists.

Is there anything I can do to handle this case?

Thanks,

Tabor

-- 
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Tabor J. Wells                                     twells@???
Fsck It!                 Just another victim of the ambient morality