Exchange Connection Problems

How to Fix Python SMTP Error 535 After Basic Authentication Is Disabled

Rana Garcia Sep 27, 2026 869 views

Question details

The user needs to resolve an SMTP Error 535 in their Python script because Microsoft Exchange has deprecated basic authentication.

How to Fix Python SMTP Error 535 After Basic Authentication Is Disabled

Product: Microsoft Exchange / Python
Device & OS: not provided
Scenario: Attempting to send automated emails via a Python script using Microsoft 365 or Exchange Online.
Observed behavior: The email script fails to execute and returns SMTP Error 535 Authentication Failed, blocking the outbound messages.

Before you start

Ensure you have administrative access to your Microsoft Azure Active Directory (Entra ID) portal to register applications and configure OAuth 2.0 API permissions.

Solution 1Recommended

Migrate Python Script to OAuth 2.0 Authentication

Update your Python email script to authenticate using OAuth 2.0 tokens rather than relying on standard username and password credentials.

Microsoft has permanently disabled Basic Authentication for Exchange Online to improve data security. To restore email sending functionality, developers must transition their automated scripts to Modern Authentication (OAuth 2.0). This process involves registering an application in Azure and generating a secure access token.

Register an application in Azure AD

Log in to the Microsoft Entra ID (Azure AD) admin center. Go to 'App registrations', click 'New registration', and configure the application name and supported account types.

Grant Microsoft Graph API Permissions

Navigate to 'API permissions' within your new app. Add a permission for Microsoft Graph, select 'Application permissions', and check 'Mail.Send'. Grant admin consent for your organization.

Generate a Client Secret

Go to 'Certificates & secrets' and create a new client secret. Copy the secret value immediately, as it will be hidden after you leave the page.

Update your Python script using MSAL

Install the Microsoft Authentication Library (MSAL) for Python via pip. Modify your script to request an access token using your Client ID, Tenant ID, and Client Secret.

Format and pass the XOAuth2 string to SMTP

Use the retrieved access token to construct an XOAUTH2 encoded string. Pass this string into your smtplib connection using the 'docmd("AUTH", "XOAUTH2 " + auth_string)' command to securely authenticate.

Migrate Python Script to OAuth 2.0 Authentication

Consult Documentation: For detailed code examples and specific implementation logic, refer to Microsoft's official documentation on Authenticated SMTP and MSAL for Python.

Solution 2

Use an App Password as a Temporary Workaround

If OAuth 2.0 implementation is currently blocked, generate a dedicated App Password if your organization's security policies still permit it.

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Frequently Asked Questions

Why did Microsoft disable Basic Authentication for SMTP?

Microsoft disabled Basic Authentication across Exchange Online to enhance security. Basic Authentication passes credentials in plain text or simple encoding, making it highly vulnerable to brute-force attacks and credential stuffing. Modern Authentication (OAuth 2.0) uses token-based verification, which is significantly more secure.

Does Python's smtplib support OAuth 2.0?

Yes, Python's built-in smtplib library can handle OAuth 2.0. However, instead of passing a regular password, you must generate an XOAUTH2 encoded string containing the user's email address and the OAuth 2.0 access token, then send it using the SMTP AUTH command.

Can I still use SMTP port 587 with OAuth 2.0?

Yes. The transition to OAuth 2.0 changes how the authentication string is processed, but the connection protocol remains the same. You should still connect to smtp.office365.com on port 587 and initiate TLS encryption using the starttls() method.

What if my Python script is running on a server without a UI?

If your script runs on a headless server, you should configure the Azure AD application to use 'Client Credentials Grant' or a background service daemon approach, allowing the script to authenticate autonomously without requiring interactive user login prompts.

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