Hi, we have the following error occurred while attempting to send the test email and the email method is smtp to an account in office365:
5.7.3 Authentication unsuccessful [VI1P18901CA0023.EURP189.PROD.OUTLOOK.COM 2023-04-13T15:54:33.075Z 08DB3AE9AE527284]
That error means that the credentials you are providing (username and password for instance) are most likely wrong
+1 for what @mnakalay said - also sometimes I’ve run into this when a client starts requiring 2FA on their Office365 install and the user they’ve created for the SMTP connection isn’t able to authenticate anymore without 2FA.