Hi @davev
Does this system@home.yourdomain.com address, respective the “home” subdomain exist on the Mail-In-A-Box server? Afaik the “From address” should be the same as the one you send the mails with and login to the SMTP server of your MiaB, which means it has to exist on your mail server. I use a MiaB myself and I created dedicacted accounts on it for each of my servers. I normaly use hostname@mydomain.tld. In your case you could use home@yourdomain.com or system.home@yourdomain.com.
Example:
Create a new user on your MiaB called e.g.: system.home and then use the following settings in Nextcloud
Send Mode: SMTP
Encryption: SSL/TLS
From address: home.system@yourdomain.com
Sever address: mail.yourdomain.com:465
Authentifiaction method: “Login” with “Authentification required”
SMTP Username: home.system@yourdomain.com