These is an issue when using MIAB only as a sender server:
For example, user1@domain.com sends a mail to user2@domain.com (or to another domain hosted on the same box).
MIAB doesn’t check the MX records of the domain, but instead delivers it to the user@ account inside the box, even if the mx record for domain.com doesn’t point to the box.
Is there a simple way to fix this issue?
Can you please more details on your question?
For example, what are you using to send the email with?
It likely can be done within Postfix configurations, but it is outside of the scope of this project, so it would be on you to discover the specific settings and change them. But be aware that MiaB will likely revert your changes upon either daily maintenance or an upgrade.
What you are describing is similar to the setting in cPanel for ‘remote’ vs ‘local’ mail exchanger.