Introduction

Outlook often starts failing SMTP authentication right after a password change because one part of the mail setup updated and another part did not. The mailbox password may be new, but Outlook, a mobile mail app, or a site relay can still be sending the old credential. Some providers also stop accepting the normal password and require an app password or a new sign-in approval step. The fix is to check the full sending path instead of changing the password again and again.

Symptoms

  • Outlook sends and receive checks fail with an authentication or login rejected error
  • Messages stay in the Outbox after the mailbox password was changed
  • Webmail works, but Outlook or another mail client cannot send
  • SMTP tests fail while IMAP access still succeeds
  • The issue started immediately after a password reset, MFA change, or security alert

Common Causes

  • Outlook is still using the previous saved password for SMTP authentication
  • The provider now requires an app password instead of the mailbox password
  • SMTP username, sender address, or authentication method does not match the account policy
  • Cached credentials in Windows Credential Manager or the mail profile are stale
  • A mailbox security control blocked sign-in from the client after the password change

Step-by-Step Fix

  1. Confirm whether the failure affects only sending or both sending and receiving, because SMTP authentication problems can look like a full mailbox outage.
  2. Sign in through the provider webmail or admin portal first so you know the new password actually works on the live account.
  3. Update the saved password or re-add the mailbox in Outlook so the client stops reusing the old SMTP credential.
  4. Check whether the provider now requires an app password, modern authentication, or a separate relay credential for desktop mail clients.
  5. Verify the SMTP hostname, port, encryption mode, and username format against the provider's current setup requirements.
  6. Clear stale saved credentials from the local system if Outlook keeps retrying the old password even after you changed the account settings.
  7. Review mailbox security alerts, sign-in logs, or blocked login notifications in case the provider flagged the client after the password reset.
  8. Send a fresh test message only after the credential and SMTP settings match, then confirm the message leaves the Outbox and reaches the destination mailbox.
  9. Document the final client settings so future password rotations or mailbox migrations do not break Outlook sending again.