If a Reply Failure has been entered for the mailbox rule, this will overwrite the actual error message which maileater is returning.
Delete the Reply Failure entries and save the mailbox rule.
Then send another email and an immediate reply should be received, except this time the actual error will be returned.
For example instead of just "FAILED" as shown in the screenshot above, something like the following would be returned:
"AHD03123 required attribute priority is missing"