Your MIDAS booking system is capable of sending email via SMTP.
If you've configured your MIDAS system to send email via Microsoft's Office 365 or Outlook SMTP servers you may encounter the following error when sending email:
Error: 535 5.7.3 Authentication unsuccessful
According to Microsoft, this error likely means that you've not enabled SMTP Authentication in your Office 365 account.
How to enable SMTP Authentication in Office 365
The per-mailbox setting to enable SMTP AUTH is available in the Microsoft 365 admin center.
Once you've enabled SMTP Authentication for the mail account you've specified in your MIDAS system, you should then be able to send email from your MIDAS system without encountering a "535 5.7.3 Authentication unsuccessful" error.