I am encountering the error "535: 5.7.3 Authentication unsuccessful [PN3PR01CA0013.INDPRD01.PROD.OUTLOOK.COM]" while using the MailKit library to send an email with Outlook's modern authentication.

I have already reviewed the document and ensured that all the necessary grants have been applied. Additionally, the required API permissions for sending emails via SMTP have been approved by the administrator. For further clarification, please refer to SMTP Rights for email for more clarification.

The following code is used to authenticate with the Outlook account:

smtpClient.Connect("smtp.office365",587, SecureSocketOptions.StartTls);

await smtpClient.AuthenticateAsync(new SaslMechanismOAuth2("[email protected]", result.AccessToken));