Add note on working MX, FCrDNS and SPF+DKIM+DMARC to documentation

[baknu]

Some inbound mail servers may check whether an SMTP EHLO domain has (1) a working MX and (2) FCrDNS. So, for the mail test it is important that the EHLO domain that is used by the Internet.nl test suite has both in place.

For the EHLO domain setting see:

Internet.nl/internetnl/settings.py

Line 97 in 7426760

SMTP_EHLO_DOMAIN = getenv("SMTP_EHLO_DOMAIN", "internet.nl") # MUST be ASCII; A-label for IDNs (i.e., xn--)

And see:

Internet.nl/docker/host-dist.env

Line 24 in 7426760

SMTP_EHLO_DOMAIN=$INTERNETNL_DOMAINNAME

Is it indeed correct that e.g. batch.internet.nl is using batch.internet.nl as the EHLO domain?

Furthermore, SPF+DKIM+DMARC should be configured. In case a domain is used for the Internet.nl test suite that has no regular outbound mail the following records should be used.

internet.nl		IN TXT	"v=spf1 a -all"	; The "a" mechanism is needed for the mail test (see rfc7208, section-2.3).
_domainkey.internet.nl	IN TXT	"v=DKIM1; p="	; empty DKIM to score 100% for this non-sending subdomain that does have SPF "a" mechanism which is needed for mail test.
_dmarc.internet.nl	IN TXT	"v=DMARC1; p=reject; sp=reject;"

The right place in the documentation to cover this seems to be: https://github.com/internetstandards/Internet.nl/blob/main/documentation/Docker-deployment.md#dns-setup

[mxsasha]

Is it indeed correct that e.g. batch.internet.nl is using batch.internet.nl as the EHLO domain?

RFC821 says "host name of the sender-SMTP", so that would suggest yes.