If you want to set up debian.net subdomain, you may setup E-mail server on your own. But if there is not afford to setup it by yourself, you need external E-mail server.
In this article, I'll explain how to use external mailbox service - Sakura Mailbox service.
I've chosen Sakura mailbox service because of maintainance cost (87yen/month)
Set dnZoneEntry, it means that gpg --clearsign and send to firstname.lastname@example.org. Here is an example for fabre.debian.net.
fabre IN A 18.104.22.168 mail.fabre IN A 22.214.171.124 fabre IN MX 10 mail.fabre.debian.net.
126.96.36.199 is Web server (https://fabre.debian.net) and 188.8.131.52 is E-mail server (Sakura mailbox service) for my case.
Note that It varies for your case.
Configure Sakura mailbox service
Set fabre.debian.net subdomain without transfer in Domain/SSL menu. Then create each account for @fabre.debian.net. After a while, you can use email@example.com E-mail account. Yay!