535 5.7.3: Authentication unsuccessful

Microsoft 365 / ExchangeSMTPSign-in problem

The error usually looks like this:

535 5.7.3 Authentication unsuccessful

What it means

The Exchange or Microsoft 365 server rejected your app’s sign-in, without giving a specific reason.

Common causes

  • Wrong username or password, or the username is not the full email address.
  • Basic authentication is disabled for SMTP, or SMTP AUTH is disabled for the mailbox.
  • For OAuth sign-ins: the access token is expired, or was issued for the wrong resource or scopes.

How to fix it

  1. Check the username and password, and that SMTP AUTH is enabled for the mailbox.
  2. If your app uses OAuth, decode its access token to check the audience and scopes.
  3. If your app only supports passwords, Auth-Email can sign in with OAuth on its behalf.

Tools to help diagnose it

OAuth token decoder and XOAUTH2 builder

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

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

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 / Exchange 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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