Roy Badami <roy(_at_)gnomon(_dot_)org(_dot_)uk> writes:
When there _is_ an upstream MTA, 5xx errors will cause backscatter;
when there isn't they won't. SPF is no different from anything else
in that respect.
If the upstream MTA (and any MTAs upstream of it) also implements SPF
(or whatever other protocol) then the 5xx error will be sent to the
originator and there would be no back scatter as the mail would be
rejected at the first relay.