Hotmail personal account: IMAP OAuth2 works but SMTP OAuth2 fails with message: 535 5.7.139 “SmtpClientAuthentication is disabled for the Mailbox”

  -> Incoming IMAP: outlook.office365.com, port 993, SSL/TLS, OAuth2, and
  -> Outgoing SMTP: smtp-mail.outlook.com, port 587, STARTTLS, OAuth2.

I am assisting an elderly user with a personal @hotmail.com account that receives email successfully in Thunderbird but cannot send. I have confirmed they can send and receive emails from their online outlook.com portal. The user's computer setup is as follows: * Windows 11 (with latest updates). * Thunderbird 153.0.2 (64-bit). * A personal @hotmail.com account with the following server configurations: -> Incoming IMAP: outlook.office365.com, port 993, SSL/TLS, OAuth2, and -> Outgoing SMTP: smtp-mail.outlook.com, port 587, STARTTLS, OAuth2. Thunderbird successfully authenticates to Microsoft using OAuth2 for incoming email. However, when Thunderbird attempts outgoing email authentication, Microsoft's SMTP server rejects the authentication. The Error Console in Thunderbird records this message: "535 5.7.139 Authentication unsuccessful, SmtpClientAuthentication is disabled for the Mailbox." -> Refer to attached image: "Thunderbird Error Console_20260811.jpg". -> I am able to provide a short screen recording demonstrating the failure if needed. To try and resolve the above, I completed the following actions: # Confirmed Thunderbird is current (153.0.2). # Confirmed SMTP is configured for OAuth2 rather than Normal Password. # Confirmed port 587 and STARTTLS. # Re-authorised the Microsoft account/OAuth connection. # Confirmed IMAP OAuth2 works. # Confirmed Outlook.com webmail can send successfully. # Reviewed Mozilla's current “Microsoft OAuth Authentication and Thunderbird in 2026” guidance, including the section covering cases where IMAP/POP works but SMTP does not. # Captured the SMTP failure in Thunderbird's Error Console. However nothing worked, so I raised the matter with Microsoft support and eventually had the matter referred to their Tier 2 team. The Microsoft case number is: '''7107041963'''. Microsoft Support confirmed that for a personal Outlook.com/Hotmail account there is no user-accessible mailbox-level SMTP AUTH setting, and advised that SMTP is enabled by default for supported clients. However, because sending from Outlook.com webmail works, Microsoft Tier 2 declined to investigate the SMTP server response further and directed me to Thunderbird support. I am able to provide a copy of the chat transcript if needed. In any event, the TLDR version IMAP works using OAuth2, but SMTP using OAuth2 fails because Microsoft's SMTP server reports SmtpClientAuthentication is disabled for the Mailbox. Microsoft Tier 2 has referred the issue to Thunderbird support. '''Questions:''' # Is this a known current issue affecting personal Outlook.com/Hotmail accounts using Thunderbird? # Is ''535 5.7.139 SmtpClientAuthentication is disabled for the Mailbox'' known to occur with Thunderbird OAuth2 even though IMAP OAuth2 succeeds? # Is there a current Thunderbird-side workaround or alternative SMTP server/configuration recommended for personal Hotmail accounts? # Would changing the SMTP server from ''smtp-mail.outlook.com'' to ''smtp.office365.com'' be appropriate for a personal Hotmail account in Thunderbird 153, or should the current Microsoft-documented server be retained? -> I note there appears to be conflicting advice on this point in previous support threads. # Given that IMAP OAuth2 authentication succeeds for the same Microsoft account, does this indicate that Thunderbird's Microsoft OAuth authorisation itself is functioning correctly, or could SMTP require a separate permission/token that may be failing? # Are there any additional Thunderbird logs or diagnostics I can provide that would help resolve this matter? ''Disclosure: I used ChatGPT to help organise my troubleshooting notes and technical details for this post. The troubleshooting, observations and diagnostic information described above are from my own work on the affected PC.''