Outlook Fix

Outlook SMTP Authentication Failed? Here's the Real Cause

Getting an 'SMTP authentication failed' or 'username and password not accepted' error trying to send mail through Outlook.com or Microsoft 365 from a third-party client, even though the password is definitely right? The cause depends on which kind of account it is, and the fix is the same for both.

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Personal Outlook.com accounts: password sending is over

For personal Outlook.com, Hotmail, Live, and MSN accounts, Microsoft turned off Basic Authentication, app passwords included, on September 16, 2024. That covers sending (SMTP) as well as reading. A client that sends with a username and password is refused even with a correct, previously working password, because Microsoft no longer accepts that method for these accounts.

Microsoft 365 accounts: still allowed, for now

Microsoft 365 work and school accounts can still send with a password (SMTP AUTH) today, but only where it's switched on. Microsoft turned SMTP AUTH off in organizations that weren't using it, admins can turn it off for a whole organization or a single mailbox, and security settings that block legacy sign-ins (security defaults or Conditional Access) refuse it too. Any of these gives the same 'authentication failed' error with a correct password.

What's coming at the end of 2026

Microsoft plans to switch password sending off by default for existing Microsoft 365 organizations at the end of December 2026. Admins can turn it back on for a while, new organizations created after that start with it off, and Microsoft plans to announce a final removal date in the second half of 2027. OAuth 2.0 sending keeps working through all of it.

The fix: sign in with Microsoft

Questions

Why does SMTP authentication fail even though my password is correct?
On a personal Outlook.com, Hotmail, Live, or MSN account, Microsoft stopped accepting passwords and app passwords on September 16, 2024. On a Microsoft 365 account, SMTP AUTH is usually switched off for your organization or mailbox, or blocked by a security policy that refuses legacy sign-ins. Either way, the password itself isn't the problem.
Is this the same issue as IMAP authentication failing?
Same root cause, different timing. Personal Outlook.com accounts lost password sign-in for reading and sending together in September 2024. Microsoft 365 lost it for IMAP and POP starting October 2022, while password sending stays available until Microsoft switches it off by default at the end of December 2026.
Can I fix this by changing my SMTP port or security settings?
No. Port and encryption settings (like SSL/TLS) aren't the issue - the sign-in method is. For a personal account no server setting brings password sending back; for Microsoft 365 only an admin can turn SMTP AUTH on, and only until Microsoft removes it.
Does Curo need a separate SMTP password to send mail through Outlook?
No. Curo sends and receives through the same OAuth connection you set up when you sign in with Microsoft - there's no separate SMTP credential to manage.
Will this SMTP change happen to my Gmail account too?
No. This change is specific to Microsoft's Outlook.com, Hotmail, Live, MSN, and Microsoft 365 services. Gmail SMTP still accepts app passwords with 2-Step Verification enabled.
Related
All resources →Proton Mail IMAP for Third-Party Clients: Why You Can't Find ItOutlook App Password Not Working? Here's Why (and the Fix)"IMAP Basic Authentication Is Disabled" on Outlook - What It MeansHow to Connect Outlook to a Third-Party Email Client (2026 Update)What Is Microsoft Modern Authentication, and Why Does My Client Need It?Notion Mail alternativeBest email for consultantsencrypted email for business

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