https://www.inmotionhosting.com/support/edu/phpbb/general-settings/smtp-setup
Actually this is probably better:
https://www.fastcomet.com/tutorials/phpbb3/smtp-configuration
But ignore what it instructs you to use for port, auth method, etc. and use the settings shown on the instructions page in your admin area. And of course, make sure that the account is set up on your MiaB.