DNS RFC References

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Posted on January 19, 2018

https://tools.ietf.org/html/rfc1123
:RFC standard for naming host names

2.1 Host Names and Numbers

The syntax of a legal Internet host name was specified in RFC-952
[DNS:4]. One aspect of host name syntax is hereby changed: the
restriction on the first character is relaxed to allow either a
letter or a digit. Host software MUST support this more liberal
syntax.

Host software MUST handle host names of up to 63 characters and
SHOULD handle host names of up to 255 characters.

Whenever a user inputs the identity of an Internet host, it SHOULD
be possible to enter either (1) a host domain name or (2) an IP
address in dotted-decimal (“#.#.#.#”) form. The host SHOULD check
the string syntactically for a dotted-decimal number before
looking it up in the Domain Name System.

Other references
DOMAIN NAMES – CONCEPTS AND FACILITIES

DOMAIN ADMINISTRATORS OPERATIONS GUIDE

 

GMAIL FAILURES

Final-Recipient: rfc822; 
Original-Recipient: rfc822;
Action: failed
Status: 5.7.1
Remote-MTA: dns; gmail-smtp-in.l.google.com
Diagnostic-Code: smtp; 550-5.7.1 [2607::::] Our system has
    detected that this message does 550-5.7.1 not meet IPv6 sending guidelines
    regarding PTR records and 550-5.7.1 authentication. Please review 550-5.7.1
    https://support.google.com/mail/?p=IPv6AuthError for more information 550
    5.7.1 . j186si6255873qkd.386 - gsmtp

Per the Google information docs.  You will need a forward AAAA record along with a SPF and PTR record.


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