ldap/sendmail: 550 5.1.1 ... user unknown intermittent failures

[Alex Vorobiev <sasha@mathforum.org>]

redhat 7.0
sendmail-8.11.6

using ldap (openldap 2.0.7) for user information

we experience intermittent failures with delivering mail to existing
accounts.  i can't seem to establish any kind of pattern.  most
failures are for local mail, meaning that addresses are never fully
qualified (such as bob, jim, etc.).
others are failures to deliver mail that comes from the outside.

all accounts are legit, all exist, the spelling and the case of
addresses are correct.  the problems are very inconsistent.

i don't see any errors in the ldap log, nor the system log.  i ran a
stress-test, mailing dozens of messages to a user account.  one out of
thirty or so would fail with the "550 5.1.1 ... user uknown" error
message.  i am guessing that under a certain load (volume of mail)
ldap lookup queries simply fail.

has anyone experienced this problem?  is there a work-around, sendmail
or ldap tuning that would resolve this problem?

any help would be appreciated,

thanks,

--sasha sashaATmathforum.org