Sendmail 8.15.2 in Ubuntu 20.04 not even trying IPv6 / 2021-03-11

I needed a virtual machine with ubuntu so I did the base installation and also configured unattended-upgrades and sendmail to get the results. But I noticed after a while I never saw any mail from that machine.

Problem soon found:

mailer=relay, pri=30131, relay=postbode.idefix.net. [82.95.196.202], dsn=4.0.0, stat=Deferred: Connection timed out with postbode.idefix.net.

The machine wasn't even trying to reach the mailserver over IPv6! On the internal network with servers it will fail over IPv4 because of the portforwarding rule for the port from the outside IP to the mailserver but I never expected an internal machine to try IPv4.

Somehow this seems default for sendmail 8.15.2 in Ubuntu 20.04. I could find someone else asking this: No IPv6 outbound from Sendmail starting with 20.04 but no answers how/why.

At first I suspected systemd-resolved as the old saying goes that all sendmail problems are caused by DNS. But disabling that didn't fix the problem.

I now have the IPv6 address hardcoded in the configuration, that works.

dnl FEATURE(`msp', `postbode.idefix.net', `25')
FEATURE(`msp', `[2001:980:14ca:1::23]', `25')

I also found out the option ResolverOptions=+WorkAroundBrokenAAAA was set but not causing this.