As for your questions: the emails seem to work just fine. You have to make sure that your IP is not blacklisted. YNH will show your that in the Diagnosis.

My #ISP does not offer to change r-DNS records, which is one of the factors they check for good IP reputation. Also, my country has a bad reputation for #spamming, so they have banned geographical networks. I have never seen my #emails arrive in their inboxes.

Security: for us we use 2FA for everything important and SSH keys. For the users is up to them to keep their stuff secure.
I have always been compromised by the weak passwords kept by the users and then the email spamming happens. #yunohost does not have #2FA yet, other applications like #nextcloud, #bitwarden, etc have it. Do you offer these services independently from Yunohost's core services?

Backups: we have a borgbase.com account and setup Borg with YNH to backup there, daily. Very easy to setup via YNH. I make sure to get an email after the backups are done to make sure they work well.
I am very impressed with the #brog as a backup solution. I will be installing it soon. Thank you for your good work.