following up on this, I am overly-excited with the system I came up with.

My original yunohost setup was working, but I became paranoid that at some unfortuitous moment my server would go down, and I would not receive an important email... or nightmares of travelling and my server is down and I don't have a computer or the ability to fix it...

I decided to follow the suggestion of benou — to create a webmail account with the exact same email addresses, and then to setup three MX records. The first MX record points to my yunohost server with priority 10, the second MX records points to my webmail server with priority 20, and the third MX record points to my webmail's backup server with priority 30 — I was excited to see that this does work as expected... if I turned off my yunohost server, and sent myself an email from elsewhere... it would indeed arrive at my webmail/"backup receiver"... I can sleep easy knowing that there is a backup receiver if my server ever goes down, and if I'm travelling and really don't have time to deal with anything, I could always just remove the first MX record, and use the webmail normally

I then configured a cron job which runs fetchmail every 15 minutes, to fetch any emails from the webmail and bring them into yunohost, in case some email accidentally got delivered to the backup receiver

In the future, if I had a close friend who also had a yunohost or a hosting collective I was a part of, I could imagine to use this system with my friend's server as the backup receiver, instead of the webmail as the backup receiver -- I think a key part of the relief of the backup, is that its another person, and something I can fallback on, if I don't feel able (something to think about in the sometimes-individualistic paradigm of self-hosting...)

of course this is all somewhat unnecessary, and just using Migadu webmail normally seems like a good option too, but this was for some reason interesting to me, and I'm happy with what I learned from it. here is a more detailed tutorial on my setup in case any future readers want to replicate it

cc @rathantara maybe this setup is interesting to you too