No Joy. I forwarded a sample bounced email as part of the support ticket. All that happened was that I got an automated reply:
I've heard nothing since and the problem is not solved - the bouncing is still happening.
It seems that the email forwarding feature in the Dreamhost Panel is half-baked if it allows forwarding but doesn't include an SRS option.
Surely this problem must come up frequently for other people: Lots of email servers/providers send using SPF and some (like Demon Internet in the UK) will automatically bounce any incoming message using SPF that doesn't have the correct sender credentials.