Re: sender rewrining advice

 My understanding is that the hash offers a modicum of security to prevent (for some value) someone reversing your SRS mechanism and sending messages to your server that your server would end up sending back out as spam.  I think that it's mostly anti-abuse / anti-reply.
 If I know your secret hash seed I could use that to generate an SRS that your system would trust, reverse the SRS and pass the message on to the intended destination as if it originated from your server.
 

Hmmm, I don't really get this. My spf (and maybe even dkim) are still applied not?
If I am processing the message
test@gmail.com -> test@example.com forwarded to test@guerrillamail.com
then my host B (outgoing) will create a new envelope, something like:
SRS0=HHH=TT=example.org=test@example.com
Any receiving host will still check the same example.com spf, as if it would be a 'regular' envelope.