You autoreply function should check if the following headers are present in the emails:
Auto-Submitted
Precedence
X-Auto-Response-Suppress
and of course NOT to generate autoresponse when these are in original message.
Then your autoreply message should have following headers set:
insert-header("Auto-Submitted", "auto-generated")
insert-header("Precedence", "bulk")
insert-header("X-Auto-Response-Suppress", "All")
The problem is that not every "autoresponder" application honors those headers. So, some of the autoresponders are using null envelope sender email address - like non-delivery-reports - to avoid loops.
