Newly registered personal Outlook email accounts cannot pass SMTP authentication when using MailKit

Question

Hello,

I recently created a new personal Outlook mailbox in 2026. The following method fails to authenticate, whereas previously created mailboxes work fine with the same approach:

using (var client = new SmtpClient())

{

client.ServerCertificateValidationCallback = (sender, certificate, chain, errors) => true;

client.Connect("smtp.office365.com", 587, SecureSocketOptions.Auto);

client.Authenticate(new SaslMechanismOAuth2("****@outlook.com", PasswordToken));

client.Disconnect(true);
}，

The PasswordToken used for OAuth2 authentication includes the following scopes:

https://outlook.office.com/IMAP.AccessAsUser.All https://outlook.office.com/SMTP.Send openid profile email offline_access;

However, when attempting OAuth2 authentication with this personal mailbox, I receive the following error:

MailKit.Security.AuthenticationException:“535: 5.7.139 Authentication unsuccessful, SmtpClientAuthentication is disabled for the Mailbox. Visit https://aka.ms/smtp_auth_disabled for more information. [SG2PR06CA0183.apcprd06.prod.outlook.com 2026-02-27T06:29:52.654Z 08DE74B70DDD0B60]”;

The linked page (https://aka.ms/smtp_auth_disabled) only explains how to enable SMTP AUTH for organizational (work/school) mailboxes in Exchange Online.

My question is: For a personal Outlook.com mailbox, where can I enable SMTP AUTH? Is it even possible?

Answer 1

For newly created personal Outlook.com mailboxes, SMTP AUTH can’t be enabled by the user in any settings UI. When SMTP client authentication is disabled on the mailbox, SMTP AUTH (including OAuth2 via smtp.office365.com) will fail with the 535 5.7.139 error that mentions SmtpClientAuthentication is disabled for the Mailbox.

The available guidance for enabling SMTP AUTH (via SmtpClientAuthenticationDisabled) applies to Exchange Online (work/school) mailboxes and is done by an administrator in Exchange Online PowerShell, either at:

• Tenant level via Set-TransportConfig -SmtpClientAuthenticationDisabled $False, or
• Mailbox level via Set-CASMailbox <EmailAddress> -SmtpClientAuthenticationDisabled $false / Set-CasMailbox <mailbox account> -SmtpClientAuthenticationDisabled $False.

These admin PowerShell options are not available for consumer Outlook.com accounts. The documentation and troubleshooting steps for SMTP AUTH (including enabling it per mailbox) are scoped to Microsoft 365/Office 365 organizational tenants, not personal Outlook.com.

Therefore, for a personal Outlook.com mailbox:

• There is no documented way for the end user to turn on SMTP AUTH if it is disabled at the service side.
• The supported path for modern authentication is to use a client that supports OAuth with the appropriate Outlook.com endpoints (for example, Outlook apps or other OAuth-capable clients as described in the Outlook.com basic-auth deprecation guidance), rather than relying on SMTP AUTH where it is blocked.

In short: SMTP AUTH enablement via SmtpClientAuthenticationDisabled is an Exchange Online (organizational) feature and is not something that can be toggled for personal Outlook.com mailboxes by the user. If SMTP AUTH is disabled for a new personal mailbox, it cannot be enabled using the documented admin methods.

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References:

• Can't connect to Outlook by using POP/IMAP and Modern authentication
• Fix issues with printers, scanners, and LOB apps that send email using Microsoft 365
• Outlook and other apps are unable to connect to Outlook.com when using Basic authentication
• Settings for IMAP4 access to Microsoft personal email accounts