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Re: [ietf-smtp] Address transformations

2016-07-31 21:25:31
(RFC 5321)
All this stuff brings up a very interesting point about deliverable email 
addresses in draft-seantek-mail-regexen:

What are the domain part limitations on deliverable email addresses, that are 
not encapsulated by the RFC 5321 ABNF?

Specifically:
The domain production is limited to 253 characters. I am sure of this.
Each sub-domain production is limited to 63 characters. I am also sure of this.

I strongly believe that domain productions cannot be parseable IPv4 addresses. 
(Note: they can never be parseable as IPv6 addresses, as that would require : 
which is not part of a domain production.)

Consider:

foo@1.2.3.4

The string “1.2.3.4” is a valid domain production. However, it is not (or 
*should not*) be considered a deliverable email address, because when passed to 
the famous function “gethostbyname”, that function will certainly return 
1.2.3.4; it will not perform a lookup of the domain record with a top-level 
label of “4”. This is also supported by RFC 1912: “Labels may not be all 
numbers”. But an individual label can be a number, such as “411.org”.

I have tried “foo@1.2.3.256” -- Windows and Unix/Linux stacks will try to query 
the DNS for that string and not parse it as IPv4. So, it is syntactically a 
valid, deliverable email address. But foo@411 will get IPv4-parsed to 
0.0.1.155, which means that “411” should not be considered a valid domain name 
for a deliverable email address.

True? Are there other limitations?

Sean


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