Processing Messages – Handler Output

The handler is expected to produce the following output on standard
error:

• PASS

  The message should be passed further. The handler should write the
  message, possibly modified, on the standard output.

• STOP

  The message should not be passed further and this is not an error.
  This should be used when the handler implements its own custom
  delivery instead of the delivery provided by the MTA. Subsequent
  handlers won’t be called.

• DEFER

  The message should not be passed further. A temporary error condition
  should be indicated. Subsequent handlers won’t be called.

• REJECT

  The message should not be passed further. A permanent error condition
  should be indicated. Subsequent handlers won’t be called.

• LOG text

  The line of text should be treated as a warning or error message and
  added to the appropriate log.

• SKIP

  The handler should not process and pass the message to the handler’s
  output. The message should be passed further without any
  modifications.

Each line of output on the standard error should be terminated with LF.
After the first PASS, STOP, DEFER, REJECT, or SKIP
line, further output is not accepted, so any LOG lines must precede the
above.

The exit code from the handler should be 0 on PASS or STOP and
not 0 in other cases.