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.