Usage Detected - Basic Auth with SMTP AUTH

As announced in MC786329, Exchange Online will permanently remove support for Basic authentication with Client Submission (SMTP AUTH).

You are receiving this post because we observed messages sent using Basic Auth with SMTP AUTH in your tenant between Sept 22 and Sept 26, 2025.

Please review the SMTP AUTH Clients Report (https://learn.microsoft.com/en-us/exchange/monitoring/mail-flow-reports/mfr-smtp-auth-clients-report) and filter on Basic Auth to determine your mailboxes that are using Basic Auth and ensure you have migrated to an alternative solution following the deprecation timeline.

We will gradually begin rejecting a small percentage of Basic Auth submissions for all tenants on March 1st 2026 increasing to 100% rejections on April 30th 2026, (previously September 2025). After this time, applications and devices will no longer be able to use Basic auth as an authentication method and must use OAuth when using SMTP AUTH to send email.

Basic auth is a legacy authentication method that sends usernames and passwords in plain text over the network. This makes it vulnerable to credential theft, phishing, and brute force attacks. To improve the protection of our customers and their data, we are retiring Basic auth from Client Submission (SMTP AUTH) and encouraging customers to use modern authentication methods that are more secure.

[When this will happen:]

We will be making this change beginning March 1st 2026 and completing April 30th 2026, previously September 2025.

[How this will affect your organization:]

The Client Submission (SMTP AUTH) endpoints in scope for this change are:

• smtp.office365.com
• smtp-legacy.office365.com

Once Basic auth is permanently disabled, any clients or apps connecting using Basic auth with Client Submission (SMTP AUTH) will receive this response:

• 550 5.7.30 Basic authentication is not supported for Client Submission.

[What you need to do to prepare:]

If your client supports OAuth, follow these steps: https://learn.microsoft.com/en-us/exchange/client-developer/legacy-protocols/how-to-authenticate-an-imap-pop-smtp-application-by-using-oauth

If your client doesn’t support OAuth and you must use Basic Auth with Client Submission (SMTP AUTH), you will need to switch to one of the following alternatives before April 2026, previously September 2025:

• If you are using basic authentication with Client Submission (SMTP AUTH) to send emails to recipients internal to your tenant, you can use Microsoft 365 High Volume Email. Please visit this site to learn more: https://learn.microsoft.com/en-us/Exchange/mail-flow-best-practices/high-volume-mails-m365
• If you have an Exchange Server on-premises in a hybrid configuration, you can use Basic auth to authenticate with the Exchange Server on-premises or configure the Exchange Server on-premises with a Receive connector that allows anonymous relay on Exchange servers. Please visit this site to learn more: Allow anonymous relay on Exchange servers

Regardless of the volume of email, if you must use Basic auth to send email with Exchange Online, then you must use one of the alternatives or a 3P solution.

We understand that this change requires some adjustments, but we believe that this is a necessary step to enhance the security and reliability of our email service and your data.