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Re: [ietf-smtp] Address literals

2016-07-31 07:23:07
On 2016-07-31 10:07:12 +0200, Alessandro Vesely wrote:
SMTP provides for:

    address-literal  = "[" ( IPv4-address-literal /
                     IPv6-address-literal /
                     General-address-literal ) "]"
                     ; See Section 4.1.3

    Mailbox        = Local-part "@" ( Domain / address-literal )

However, it is not clear how mailboxes containing an address literal are to
be treated by an MTA when they appear in the envelope.  Specifically, I have
three questions, assuming a recipient is indicated using an address literal:

I asked a similar question about CNAME records about 2 years ago, and
the consensus was that a relaying MTA must not change the recipients
address while the receiver MTA may treat the address any way it pleases.

I think it is safe to generalize that to address literals.

1. If the host can resolve the address and obtain a domain name, it can
replace it so as to obtain a regular mailbox.  Right?

No. If a relay receives an email for <alice@[]>, it cannot
rewrite it to <alice(_at_)example(_dot_)com>, just because it finds a PTR record
« PTR».

Of course the receiver MTA may deliver mail for <alice@[]> and
<alice(_at_)example(_dot_)com> to the same mailbox, and it might do that
canonicalizing the domain part first. But that's local policy.

2. If the host cannot resolve the address, it can either try to relay it or
try to deliver it locally.  What is the recommended behavior?

In any case the MTA needs to determine whether it is the receiver or a
relay. If it is the receiver it must try to deliver it locally (of
course "local delivery" might result in forwarding the to a different
Otherwise it must relay it by connecting to the specified address.

Determining whether the MTA is the receiver is local policy. There is
probably no general rule like "does this IP address match one of this
host's interfaces?"

3. If in case (2) the server relays, it can either drop or keep the domain
part when it issues RCPT TO to its peer.

No. It must not change the recipient address, and just dropping the
domain part would in any case result in a syntactically invalid address.


   _  | Peter J. Holzer    | A coding theorist is someone who doesn't
|_|_) |                    | think Alice is crazy.
| |   | hjp(_at_)hjp(_dot_)at         | -- John Gordon
__/   | |

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