MTA-STS

https://www.security.gov.uk/guidance/email-guidance/mta-sts/set-up
cat /var/nsd/zones/master/ircnow.org
mta-sts 3600    IN      CNAME   www.ircnow.org.
_mta-sts 3600   IN      TXT     "v=STSv1; id=20221012"
_smtp._tls 3600 IN      TXT     "v=TLSRPTv1;rua=mailto:postmaster@ircnow.org"


mkdir /var/www/htdocs/.well-known
# ls -l /var/www/htdocs/.well-known
-rw-r--r--  1 www  daemon  63 May  1 12:53 mta-sts.txt
ircnow# ls -l /var/www/htdocs/.well-known/mta-sts.txt 
-rw-r--r--  1 www  daemon  63 May  1 12:53 /var/www/htdocs/.well-known/mta-sts.txt
ircnow# cat /var/www/htdocs/.well-known/mta-sts.txt   
version: STSv1
mode: testing
mx: mail.ircnow.org
max_age: 3600

        location "/.well-known/" {
                root "/htdocs"
        }