Will Microsoft 365 SMTP Basic Authentication Stop Working?
Question details
A user needs to know if a legacy .NET Web Forms application can continue using SMTP Basic Authentication to send emails through Microsoft 365 after Microsoft's broad deprecation of Basic Authentication.
- Product
- Microsoft 365
- Device & OS
- not provided
- Scenario
- Maintaining email functionality for a legacy .NET application that authenticates and sends messages to external addresses using Microsoft 365 accounts.
- Observed behavior
- The application's email delivery logic may fail if SMTP Basic Authentication is globally disabled without the ability to keep it enabled or without transitioning to modern authentication.
Verify with your Microsoft 365 tenant administrator regarding your organization's active security policies, and ensure you have sufficient permissions or access to Exchange Online PowerShell if configuration changes are necessary.
Verify and Re-enable SMTP AUTH for Specific Mailboxes
While Microsoft has disabled Basic Authentication across Exchange Online, tenant administrators can sometimes selectively re-enable SMTP AUTH for specific mailboxes used by legacy applications.
Microsoft's security defaults actively block Basic Authentication, but if your tenant does not have security defaults strictly enforced, you can manually allow SMTP client authentication for the designated application mailbox.
Using Exchange Online PowerShell is the most reliable way to check and modify this setting.
Ask your Microsoft 365 tenant administrator if SMTP AUTH is currently enabled for the affected application mailboxes or if organizational policy permits it to be turned back on.
If you have administrative permissions, open PowerShell and run 'Connect-ExchangeOnline' to authenticate with your Microsoft 365 admin credentials.
Run the command 'Get-CASMailbox -Identity application@yourdomain.com | Format-List SmtpClientAuthenticationDisabled' to see the current status.
If disabled, run 'Set-CASMailbox -Identity application@yourdomain.com -SmtpClientAuthenticationDisabled $false' to allow Basic Authentication specifically for that account's SMTP submissions.
Update Application to Use Modern Authentication (OAuth 2.0)
If Basic Authentication is permanently disabled or blocked by security policies, you must update the application code to utilize OAuth 2.0 for Microsoft 365 email submission.
Configure a Microsoft 365 SMTP Relay for External Recipients
If updating the application code is not immediately possible, you can set up a Microsoft 365 connector to relay emails based on the application server's IP address.
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Frequently Asked Questions
What is Microsoft 365 SMTP Basic Authentication?
SMTP Basic Authentication is a legacy method where an application sends a username and password in plain text (encoded) to an email server to verify its identity before sending an email. Microsoft is deprecating this across Microsoft 365 in favor of more secure, token-based Modern Authentication (OAuth 2.0).
Can I still send emails to external addresses using Microsoft 365 if Basic Authentication is disabled?
Yes. If your legacy application cannot be updated to OAuth 2.0, you can configure an SMTP relay using an inbound connector in the Exchange Admin Center. This allows your application to authenticate via its static public IP address rather than using a username and password, permitting mail delivery to external recipients.
How do I get help updating my legacy .NET code for Office 365 modern authentication?
If you are migrating from SMTP Basic Authentication to OAuth 2.0 or the Microsoft Graph API, you can consult the official Microsoft documentation for the Microsoft Authentication Library (MSAL) or ask specific implementation questions in the Microsoft Q&A Office developer forums.
