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Fix "535 5.7.3 Authentication unsuccessful" email errors

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.

  1. Open your Microsoft 365 admin center and go to Users → Active users.
  2. Select the user, and in the flyout that appears, click Mail.
  3. In the Email apps section, click Manage email apps.
  4. Verify the Authenticated SMTP setting: unchecked = disabled, checked = enabled.
  5. When you're finished, click Save changes.

For further information, please see this post from Microsoft's forums.

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.

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MIDAS » KB » Support » Article 00250

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