For all intents and puposes, the spammer MTA is the MSA. There must be an MSA.
Ground rules for my hypothetical environment.
1. If a received email does not contain a return-path header, an MTA should
create a return-path header based on the envelope sender.
2. If a received email does contain a return-path header, an MTA should not
alter it or create any additional return-path headers.
3. MTAs should send DSNs to the recipient specified in the return-path header.
4. Forwarders should use the local mailbox as the envelope sender on forwarded
email.
1 & 2. I mean, any MSA/MDA/MTA. If the envelope sender is forged, then the
trace path was broken to begin with.
3 & 4. The DSN would go to the forged return-path. This is already the status
quo.
DSN backscatter is already a part of the standard. Every time a spammer sends
an e-mail mail with a forged envelope sender that results in a delivery
failure, it is causing DSN backscatter to that forged address.
The return-path header is the most useless header for tracing an e-mail. In a
spam, the return-path header is probably forged. A return-path header of,
"spammer(_at_)jlkjasnafenjf(_dot_)com", won't help you trace anything.
The only reliable header is the received header correlating with the MX record
of the original recipient domain. The one the says something like, "received:
from [spammer] by [MX for original recipient domain]".
IPs already get RBLed for DSN backscatter to forged addresses. If the SPF
header were trustworthy, then it could have value. If the spammer MSA is
sending e-mail directly to the recipient MDA and the MDA performs an SPF check
and creates an SPF header, then that information can be relied upon.
If the return-path header is written by the MSA, then forwarders can use the
local mailbox as the envelope sender and they will pass SPF. SPF implementation
will decrease forgery, hence decreasing DSN backscatter to forged addresses.
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