The EX_UNAVAILABLE error
code (value 69) indicates that some system resource is unavailable.
Several examples follow.
If the body size of an incoming message is larger than the size limit
imposed by the M= equate (M=), the following error is reported:
Message is too large; M=bytes max
All delivery agent programs must be designed to return an exit value
that is defined in <sysexits.h>. If a
poorly designed delivery agent exits with some other value,
sendmail will issue this error and consider the
delivery to have failed:
unknown mailer error decimal representation of the error value
When delivering to a file, sendmail forks, and
the child performs the actual delivery. If that child dies because of
a signal, the parent logs this error:
child died on signal decimal representation of the signal value