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[ietf-smtp] Domain names in the presence of CNAME records

2013-05-06 04:13:25
Hello everybody,

this is a somewhat theoretical topic, but it seems to come up every few
years in usenet discussions, so I'll ask for clarification/opinions.

Consider this DNS configuration: CNAME MX    10 A

Now an MTA receives a message addressed to 
<user(_at_)A(_dot_)example(_dot_)com>. This
MTA is a relay system and has no special configuration with regard to

RFC 5321, section 5.1 specifies how to resolve "", so by
following the chain -> -> the MTA
will end up connecting to We now have an SMTP connection and
I will call this MTA the client and the server.

Which address should the client use in the RCPT TO command?

1) <user(_at_)A(_dot_)example(_dot_)com>, because a relay must pass on the 
2) <user(_at_)M(_dot_)example(_dot_)com>, because is the 
canonical name 
   of the recipient domain.
3) Undefined behaviour. The MTA could even have rejected the message.
4) Other.

If the client sends "RCPT TO:<user(_at_)A(_dot_)example(_dot_)com>", how should 
server treat this?

1) Exactly the same as "RCPT TO:<user(_at_)M(_dot_)example(_dot_)com>", because is an alias for, so both must behave the
2) <user(_at_)A(_dot_)example(_dot_)com> and 
<user(_at_)M(_dot_)example(_dot_)com> may designate different 
   mail boxes (and only one of them may exist).
3) Other.

I do have an opinion and I think I can back it up with quotes from the
RFC, but I've tried to state the question neutrally. 


   _  | Peter J. Holzer    | Der eigene Verstand bleibt gefühlt messer-
|_|_) | Sysadmin WSR       | scharf. Aber die restliche Welt blickt's
| |   | hjp(_at_)hjp(_dot_)at         | immer weniger.
__/   | |   -- Matthias Kohrs in desd

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