I’ve disabled catch-alls in the admin for now since they have odd behavior. Until that’s sorted out, you can add them with the tools/mail.py command-line tool.
You want to send an email to everyone at a domain? Pretty sure mailinabox doesn’t support that feature.
Easiest is to just compose an email and address it to all your users.
If you’re ready for pain and learning, you could manipulate Postfix directly… Here’s a post, but step 1 needs to use /home/user-data/mail/mailboxes/DOMAIN instead of /home.