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Outlook.com SMTP OAuth2 authentication fails with error 535 despite successful sign-in

Thunderbird Send and receive email
ivanyuanjunior
ivanyuanjunior

My personal Outlook.com account can receive emails normally through Thunderbird IMAP, and sending emails through Outlook.com webmail also works.

However, SMTP sending in Thunderbird fails even though Microsoft OAuth2 sign-in completes successfully.

SMTP configuration:

  • Port: 587
  • Security: STARTTLS
  • Authentication: OAuth2

Thunderbird's Error Console shows:

  • 535 Authentication unsuccessful
  • Error during AUTH XOAUTH2
  • 500 Unrecognized command 'Length: 7'

I have verified the settings and refreshed my OAuth2 credentials. The error persists.

This appears identical to Mozilla Bugzilla issue #1917321.

I contacted Microsoft Support (case #7111097598), but the agent referred me to Thunderbird without confirming whether SMTP AUTH is enabled for my mailbox.

Could someone help determine whether this is an OAuth2 configuration issue, a Thunderbird compatibility problem, or a Microsoft-side SMTP restriction?

My personal Outlook.com account can receive emails normally through Thunderbird IMAP, and sending emails through Outlook.com webmail also works. However, SMTP sending in Thunderbird fails even though Microsoft OAuth2 sign-in completes successfully. SMTP configuration: * Server: smtp-mail.outlook.com * Port: 587 * Security: STARTTLS * Authentication: OAuth2 Thunderbird's Error Console shows: * 535 Authentication unsuccessful * Error during AUTH XOAUTH2 * 500 Unrecognized command 'Length: 7' I have verified the settings and refreshed my OAuth2 credentials. The error persists. This appears identical to Mozilla Bugzilla issue #1917321. I contacted Microsoft Support (case #7111097598), but the agent referred me to Thunderbird without confirming whether SMTP AUTH is enabled for my mailbox. Could someone help determine whether this is an OAuth2 configuration issue, a Thunderbird compatibility problem, or a Microsoft-side SMTP restriction?
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