E-mail self-hosting with freedombox

I have been running Freedombox (see https://www.freedombox.org/) on the so-called "Pioneer box" (Olimex Olinuxino-Lime2, running u-boot and Debian using main repository, so blob-free as far as I understand) for 1,5 years now using ejabberd, radicale, quassel and openvpn.

I tried to activate postfix/dovecot for email, I just clicked on "enable" and followed the instructions for port redirection and DNS records (not exactly for DNS records, if you are using a subdomain, you need to add it as suffix in the "domain" field of each record, unlike the advice, and I had to remove the quotes in the middle of the DKIM records) and it works. The most difficult was setting up the DNS records with my DNS provider, and this is using non-free JS unfortunately.

This is only one box doing it all, with just an SD card as storage (but you can use the version with an SSD), so the reliability is limited (I lost control of it 3 or 4 times, of course it can happen when you are away and can't do anything), but still, this is almost effort-less self-hosting with a tiny low power silent machine and free software only.

I'll investigate if there is a way to have two boxes such that one takes over if the other is unavailable.