From to the link you posted:
"The patch arrived in OpenSMTPD 6.6.2 (6.6.2p1 if you are using the so-called
Portable source code intended for use on operating systems other that OpenBSD
itself)."
In Trisquel 8.0 LTS Fildas, the latest version is 5.7.3p2-1 amd64, according
to my apt search. If this is what you are using then I would be
extraordinarily concerned, just as you say.
In this case, you can either wait for it to be updated for Fildas or do it
yourself if you have the ability (I don't myself, but maybe you can find
someone who can help you in this forum).
Otherwise, I would suggest installing the Guix package manager (as it already
has OpenSMTPD 6.6.2p1: https://guix.gnu.org/packages/opensmtpd-6.6.2p1/) and
then use the guix pack utility to create a binary tarball that includes all
dependencies built in, or it can make a docker image for you...either of
which you can use on your current system
(https://guix.gnu.org/manual/en/html_node/Invoking-guix-pack.html#Invoking-guix-pack).
For full disclosure, I've never done this before and only know it is an
option that should work according to the Guix guys, but honestly, I don't
know that this would be any faster than just compiling the new version
yourself and making a package for it that will work with apt.
Another thought is to just add a ppa to apt that already has it and install
it from there...although I don't know that the dependencies would be the
same.
Whichever way you decide to handle it, I would shut down that mail server
immediately if it accepts random emails from the internet. You aren't likely
to be able to easily recover from any new outgoing spam activity if you
already have a history of it.
Best of luck,
Platoxia