Re: path routing

John R Levine wrote:
 
> Oh, wow, path routes.  There's a quixotic quest.

As far as current Internet Standards are quixotic:

| the reverse-path is a return route (which may be used to
| return a message to the sender when an error occurs with
| a relayed message).

They didn't invent 551 only for the fun of more error codes.

> It most definitely does NOT say that if one of the relays
> replaces the recipient address with a new address that it
> should hang a reverse path on the return address.

It does in 3.1 MAIL:

| It gives the reverse-path which can be used to report errors.
[...]
| The <reverse-path> can contain more than just a mailbox.  The
| <reverse-path> is a reverse source routing list of hosts and
| source mailbox.  The first host in the <reverse-path> should
| be the host sending this command.
     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
> the DNS was new, and MX records were far from universal.

Sure, and they had no spam and no forged MAIL FROM addresses.

> in lacking MX, you just had to know that the only way to get
> mail to one host was through another host that happened to
> have a gateway

If you'd say that forwarding from one MX to another unrelated
MX is a waste of resources and bandwidth, then I won't disagree.
 
> Once the gateways were all documented by MX records, path
> routes became useless, which is why they're gone from 2821.

As we see that was a dangerous error, it created the loophole,
now fixed by SPF / RMX / BATV / SES / etc. as far as possible.

> Unless forwarding hosts remember every piece of mail they've
> ever forwarded, they're not going to be able to tell virtuous
> bounces actually returning along a previously used path from
> random spam and blowback that happens to have scraped that
> route from somewhere.

If they don't know how to handle it they shouldn't forward the
mail, but reject it with 551 or a simple "user unkown".  It's
the problem of the receiver to get it right.

> BATV does some swell things, but routing is not one of them.

It allows to identify bogus bounces caused by forged MAIL FROM
addresses, doesn't it ?
                       Bye, Frank