release of sendmail offers more and better ways
to handle queue problems. They are mostly implemented as options.
Table 24-7 in Section 24.7.2 lists
all options that affect the queue. Of special interest are the
Timeout.queuewarn options. Whenever you upgrade to
a new sendmail release, be sure to read the
RELEASE_NOTES for information about new ways to
solve queueing problems.
The queue directory should never be shared among machines. Such sharing can make detection of orphaned locks impossible. Bugs in network-locking daemons can lead to race conditions in which neither of two machines can generate a queue identifier.
In old versions of sendmail it was possible for
lf file to be left in place even though its
corresponding mail message was not being processed. Such spurious
files prevented the message from ever being delivered unless they
were removed by hand. Spurious lock files could be detected by
watching the syslog(5) file for frequent
Homespun programs and shell scripts for delivery of local mail can fail and lose mail by exiting with the wrong value. In the case of a recoverable error (a full disk, for example) they should exit with EX_OSERR or EX_TEMPFAIL. Both of these exit values are defined in <sysexits.h> and cause the message to be re-queued.
Because sendmail does a chdir(2) into its queue directory, you should avoid removing and re-creating that directory while the sendmail daemon is running. ...