Re: Source routing -- why not?

On Tue, Nov 30, 2004 at 10:43:40AM +0000, Tony Finch wrote:

> > After further deliberation about source routing and SPF, I have come
> > around to the conclusion that Frank is to some degree right, and if
> > you want to use SPF/Sender-ID, you should use source routes.
> 
> Source routes require all mail servers to be open relays.

Why?

If a@a sends mail to b@b, and if b@b forwards to c@c, then only
c@c can send a bounce via b@b towards a@a.

MTA 'b' accepted responsibility for the message.  MTA 'b' needs to
deliver it to MTA 'c' because user b@b wants it to do so. From that
moment on, the conversation is between 'b@b' and 'c@c', errors should
go to 'b@b' who can, if needed, send a bounce to 'a@a'.

Alex