Hi,

I read the thread from Paul Fernout and the answer from 
Janko. This is all heavy weight especially the PGP/SMIME
approach. I want to bring your attention to something that
is already existing and deals with the actual problem.

It is called Sender Policy Framework (SPF)[1] . It is basically
a method to be able to specify from which machine mails from
a certain domain are allowed to be sent.

My domain as an example has

norbert at cineflux:~$ dig TXT hartl.name

<snip/>

;; ANSWER SECTION:
hartl.name.		2018	IN	TXT	"v=spf1 mx -all"

So if I am sending a mail that is SPF aware it will check 
back this record and the record tells that my configured 
mail host is the only one allowed to send mail for that domain.
Fits quite good in the subscription system like this mailing
list.

FYI,

Norbert