535 5.7.139: Basic authentication is disabled

Microsoft 365SMTPSign-in problem

The error usually looks like this:

535 5.7.139 Authentication unsuccessful, basic authentication is disabled.

What it means

Microsoft 365 rejected your app’s username and password because basic authentication (signing in with just a password) is switched off for SMTP. Your app must sign in using OAuth 2.0 instead.

Common causes

  • Microsoft has retired basic authentication for SMTP AUTH in Exchange Online, or your organization has disabled it.
  • Your app, printer, scanner or script only supports signing in with a username and password.

How to fix it

  1. Update your app to a version that supports OAuth 2.0 (sometimes called Modern Authentication) for SMTP.
  2. If your app or device cannot be updated, use Auth-Email: it signs in to Microsoft 365 with OAuth for you, while your app keeps using a normal username and password.
  3. For devices that only send to internal recipients, Microsoft’s alternatives are direct send or an SMTP relay connector, which do not require sign-in but have other limitations.

Tools to help diagnose it

Email server checker

Check any IMAP, POP or SMTP server’s encryption, certificate and supported sign-in methods

SMTP connection tester

Spin up a real SMTP test server and diagnose your email sending configuration in real time

OAuth token decoder and XOAUTH2 builder

Decode a Microsoft or Google access token, check its scopes, and build or decode XOAUTH2 strings

Can’t update your app to use OAuth?

Auth-Email fixes sign-in errors like this one without changing your app, printer, scanner or script. It signs in to Microsoft 365 with OAuth on your app’s behalf, while your app keeps using a normal username and password over IMAP, POP or SMTP.

Read the docsTry Auth-Email →

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