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