Re: loop homework, not an experiment, draft-crocker-email-deliveredto

On 07/08/2021 16:58, John R Levine wrote:
>>> When mail systems do internal forwards using things like .forward or
>>> /etc/alias, each forward adds a Delivered-To, which breaks a->b->c->a
>>> loops.  Since there's no SMTP involved, there's usually no Received and
>>> never an Envelope-To.  Also, as Sam, Viktor, and I have explained, the
>>> internal forwards are not necessarily to or from anything that has an
>>> externally valid envelope address.
>>
>> What happens when there are multiple recipients? Message splitting?
> 
> For Delivered-To loop breaking it doesn't matter since it's just a token to show that the message has already been wherever it is.
> 
> I believe that Exim does one delivery at a time so its Envelope-To only has one address.  I can ask Jeremy what happens with multiple recipient messages.

Some Exim transports (the module that does delivery) can handle
multiple addresses, some only one.

The docs say:
"If this option is true, an Envelope-to: header is added to the message.
This gives the original address(es) in the incoming envelope that caused
this delivery to happen. More than one address may be present if the
transport is configured to handle several addresses at once, or if more
than one original address was redirected to the same final address."

The controlling option is false as default.  I don't know what distros
configure, though.


I think that means that a transport restricted (*) to one-address handling
would only add one address in an Envelope-To, but that isn't always
the case.
-- 
Cheers,
   Jeremy

*) "rcpt_max" for an smtp transport controls, defaults to >1.
    "batch_max" for appendfile, lmtp or pipe transport controls,
    defaults to 1.

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