Michael Deutschmann wrote:
With [fm=] options, senders would then be free to use "-all" in all
cases where it is appropriate by the original intention of the standard.
The original intention of the standard is to compute whether a
server is authorized to use the domain name in the sender's address.
during SMTP transactions. Since forwarding is part of that, it is
included in that intention. IOW: I think it's very difficult to
specify when a message is being "forwarded"; therefore it is
difficult to tell the difference between "fm=soft -all", and, say,
"fm=hard ~all".
A receiver that believes it has whitelisted all forwarders [...]
For the same reasons as above, such belief is necessarily heuristic.
Forwarders generally don't declare their existence before using an
email address (as they should, according to privacy norms that
require explicit consensus for such use.) When they send, there is
no way to automatically distinguish them from other transmitters
that don't pass SPF checks for their own reasons.
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