It seems that Sendmail 8.9.x is triggering some bugs in the db 1.85
implementation in NetBSD's libc, which causes high load systems
(particularly those with virtusertable, domaintable, and other additional
db's) to seemingly declare some users "unknown" and bounce random mail.

I don't know the exact cause of the bug, but it is not possible to compile
Sendmail 8.9.x reliably against NetBSD's libc using either -DNEWDB or -DNDBM
(the ndbm interface of db 1.85 does not have a dbm_pagfno(), which Sendmail
requires).  It seems that you _must_ use DB 2.x with Sendmail 8.9.x for
reliable operation.

I will attempt to get more information on this.

-- 
-- Todd Vierling (Personal tv@pobox.com; Bus. todd_vierling@xn.xerox.com)