Is null return path reliable indication of a bounce?

I have a mailbox that receives bounces as well as normal email (the latter includes messages from automated services). I want to filter out the bounces and forward them to another mailbox. Assume I cannot change the envelope return path in email I send.

Is “null return path” a reliable criterion to tell bounces from normal mail? Roughly how many false positives and false negatives can I expect if I apply it?

depends on your definition of 'bounce', if you mean an error message (NDR), then no, not nonly NDRs have an empty envelope sender/return path

• out of office / auto-replies
• spam
• DSNs with a successful delivery report
• forwarders which want to make sure they cannot backscatter
• some strangely configured web forms

depending on your environment you can probably ignore most if not all of these (eg if you don't request DSN's, have forwarders etc to that account...)

edit:// false negatives: checking my maillogs, there are apparently some broken anti-spam appliances which send NDRS with envelope sender 'postmaster@....', also, forwarded bounces can lead to the null sender being replaced by the postmaster address of the forwarding server. but the amount seems negligible