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