MX Record Answer Contains Same Domain

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What does it mean for an
MX record to have an answer section that contains itself? My
earlier belief was that this implies that a domain is it's own mail domain, but from
running a couple experiments on web domains, I get connection timeouts when doing
SMTP scans on domains that have MX
records as below.

Which RFC / where in an RFC
contains this specification?

$ dig
            -t mx yahoo.net 

; <<>> DiG 9.11.3-1ubuntu1.7-Ubuntu
            <<>> -t mx yahoo.net 
;; global options: +cmd 
;; Got
            answer: 
;; ->>HEADER<<- opcode: QUERY, status: NOERROR, id: 29654
            

;; flags: qr rd ra; QUERY: 1, ANSWER: 1, AUTHORITY: 0, ADDITIONAL:
            1 

;; OPT PSEUDOSECTION: 
; EDNS: version: 0, flags:;
            udp: 65494
;; QUESTION SECTION:
;yahoo.net. IN
            MX

;; ANSWER SECTION:
yahoo.net. 1800 IN MX 0
            .


;; Query time: 175 msec
;; SERVER:
            127.0.0.53#53(127.0.0.53)
;; WHEN: [REDACTED] UTC 2019
;; MSG SIZE
            rcvd: 53

Other domains
that have MX records like this:

umblr.com, google.az,
ardmediathek.de, huffingtonpost.ca,
yimg.com,
healthdirect.gov.au

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class="normal">Answer

First
off, as a general observation, the name . does not refer to
"the domain itself" but to the root of the DNS
tree.

Specifically in the context of
MX, though, href="https://tools.ietf.org/html/rfc7505#section-3" rel="nofollow noreferrer">the
rdata 0 . has been defined to mean "null", as in "I
do not want mail delivered".

No
MX, on the other hand, has a completely different meaning;
try delivering to the address directly associated with this name
instead (in practice this is often some web server).