Why do you expect the unwashed heathen masses to upgrade
If they install SPF checks _and_ add SPF-Received header
fields _and_ manage to shuffle these header fields before
the mail arrives in your mailbox tell them to stop this.
Or as RfC 3834 put it for the case of a missing Return-Path:
| It is better to fix the problem with the mail delivery
| system than to rely on heuristics to guess the appropriate
| destination of the response. Such heuristics have been
| known to cause problems in the past.