How to stop Sendmail sending mail from IPv6 instead of IPv4

You can do something like this:

LOCAL_CONFIG
# File that contains recipients usernames that interest us in this test
F{Users} -o /etc/mail/users-to-test
# File that contains recipients domains that interest us in this test
F{Domains} -o /etc/mail/domains-to-test

LOCAL_RULE_2
R $={Users} < @ $={Domains} . >      $#discard $: discard

/etc/mail/users-to-test contains the usernames that we do not want them to receive email from the test server. This addition to Ruleset 2 discards all email directed to them, and it is assumed that the above ruleset is applied to the sendmail running in your test machine and not on the production server.

PS: tabs separate the left from the right hand side of the rules. tabs and not spaces, so do not copy-paste.

You need to use echo with the 'e' parameter.

 -e  
    Enable interpretation of the following backslash-escaped characters in each STRING:  
      \n          new line  
      \r          carriage return

If you look at the result of sending mail with your command you see the following:

Content-Type: text/plain\r\nFrom: do-not-reply@mydomain.com\r\nSubject: Test\r\n\r\nThe body goes here

(It literally shows up as above as a single line, including the 'special characters')

Applying the above, slight, modification allows your code to work fine:

echo -e "Content-Type: text/plain\r\nFrom: do-not-reply@mydomain.com\r\nSubject: Test\r\n\r\nThe body goes here" | sendmail -f do-not-reply@mydomain.com admin@mydomain.com