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EX_TEMPFAIL (cannot lookup user through NSS, may be because of an unreachable NIS or LDAP server), or EX_OSERR (other file or memory related problems). Sendmail and Postfix should honour EX_TEMPFAIL and EX_OSERR by queueing the message. This avoids problems with transcient NSS errors, where the MTA may successfully lookup a user at a time while mail.local can fail a few secoonds later.