Unable to Send Email from Personal Outlook.com Account via SMTP OAuth2 – "SmtpClientAuthentication is disabled for the Mailbox

I would like to provide additional information because this no longer appears to be an isolated client configuration issue.

Summary

For the affected Outlook.com account:

• IMAP with OAuth2 works.
• Webmail sending and receiving works.
• SMTP with OAuth2 fails.
• The SMTP server returns:

Plain Text

535 5.7.139 Authentication unsuccessful,

SmtpClientAuthentication is disabled for the Mailbox.

Visit https://aka.ms/smtp_auth_disabled for more information.

Show more lines

Why I believe this is mailbox-specific

The same Thunderbird installation is configured with multiple Outlook.com accounts.

AccountIMAP OAuth2SMTP OAuth2Older Outlook.com account #1WorksWorksOlder Outlook.com account #2WorksWorksNewly created Outlook.com accountWorksFailsThis suggests:

• OAuth authentication itself is functioning.
• The client configuration is functioning.
• The issue is specific to this mailbox (or a class of mailboxes), not the client.

Similar reports

Microsoft Q&A

Another user reported the same mailbox-specific error on a personal Outlook.com account:

"535 5.7.139 Authentication unsuccessful, SmtpClientAuthentication is disabled for the Mailbox" Not able to send mail from personal Outlook mailbox

GitHub (email-oauth2-proxy)

A user testing a free Outlook account reported the same SMTP OAuth2 failure:

"Authentication unsuccessful, SmtpClientAuthentication is disabled for the Mailbox"

In the discussion, another user reported encountering the same issue. GitHub Discussion #405

Thunderbird/Mozilla Documentation

Mozilla's current Thunderbird documentation states:

"In some cases, SMTP authentication has been completely disabled."

and

"For new accounts, SMTP always starts disabled."

The article also notes that there are SMTP restrictions that are "not currently understood." Mozilla Thunderbird OAuth documentation

Request

Can Microsoft confirm whether:

1. SMTP AUTH via OAuth2 is currently disabled or restricted for some Outlook.com consumer accounts?
2. There is a mailbox-level flag that can produce the error "SmtpClientAuthentication is disabled for the Mailbox" on personal Outlook.com accounts?
3. This behavior is expected, a provisioning issue, or a service-side defect?

At this point I have not found any consumer-accessible setting that controls SMTP AUTH for the affected Outlook.com mailbox, and the evidence above suggests the issue is occurring after successful OAuth authentication and is specific to the mailbox rather than the email client.

Thank you.

Hi Lnx Repaircafe,

I understand that this personal Outlook.com account receives messages successfully in Thunderbird through OAuth2, but sending fails with the message "SmtpClientAuthentication is disabled for the Mailbox", while sending through Outlook.com on the web still works.

Please note that this is a user-to-user forum, so contributors cannot access the mailbox or review its server-side SMTP status. First, confirm that the affected account has its own outgoing server entry in Thunderbird:

• Server name: smtp-mail.outlook.com
• Port: 587
• Connection security: STARTTLS
• Authentication method: OAuth2
• Username: The complete Outlook.com email address

These are the supported SMTP settings for a personal Outlook.com account. POP, IMAP, and SMTP settings for Outlook.com.

Also make sure that this exact SMTP entry is assigned to the affected account under Thunderbird > Account Settings > Outgoing Server (SMTP). If necessary, remove and re-add the affected account so Thunderbird can create a fresh OAuth connection. Thunderbird supports OAuth2 for both incoming and outgoing Outlook.com connections.

If the settings are already correct and the same message continues, the rejection appears specific to this mailbox. Since there is no corresponding SMTP AUTH setting available in the Outlook.com web settings, please contact Outlook.com Support so the account can be reviewed.

1. Go to Outlook.com and sign in.  
2. Select the Help icon, the question mark, in the upper-right corner. 
3. In the Help pane that opens, search for "cannot send emails", then press Enter.   
4. Scroll down the page until you see "Still need help?", click Yes.   
5. Choose "Chat with a support agent in your web browser."  
6. Review the issue description and confirm an email address you can access. 
7. Click Confirm to create the service request. The screen will indicate that your request has been submitted and the system is connecting you to Outlook.com live chat support, which may take a moment to load.  

The available wording or support options can vary by region and support availability. The official support route is also explained here: How to get support for Outlook.com | Microsoft Support.

For privacy, do not post the complete email address, passwords, OAuth tokens, or diagnostic logs containing account information in this public thread.

I hope Outlook.com Support can clarify the mailbox-specific SMTP rejection and provide the appropriate next step.