Outlook 2010 0x800CCC91 error - plaintext authentication disallowed

A user who uses Microsoft Office Professional Plus 2010 as his email client reported that he could no longer download email. Whenever the user would check his email, a window would appear prompting him to re-enter his userid and password. Even after I reset his password on the server, the same thing kept happening. At the bottom, right-hand corner of the Outlook window, I saw a "Send/Receive error" messsage next to a yellow triangle with an exclamation mark within it.

Outlook 2010 send receive error

When I clicked on that error message, I saw "reported error (0x800CCC91): 'Your e-mail server rejected your user name. Verify your user name for this account in Account Settings. The server responded -ERR [AUTH] Plaintext authentication disallowed on non-secure (SSL/TLS) connections.'"

Outlook 2010 plaintext
authentication disallowed

To correct the problem, I changed the Outlook settings for his account to use encrypted authentication by the following steps:

  1. I clicked on File on the menu bar at the top of the Outlook window.
  2. I clicked on Account Settings.
    Generic Category (English)120x600

    Outlook 2010 Account Information

  3. I then clicked on his email account under "Name" to select it and then clicked on Change.

    Outlook 2010 - E-mail Accounts

  4. At the Change Account window, I clicked on More Settings

    Internet E-mail Settings

  5. I then clicked on the Advanced tab in the Internet E-mail Settings window.

    Internet E-mail Settings

  6. I then changed the "Incoming server (POP3)" value from 110, the well-known port for Post Office Protocol version 3 (POP3) connections to 995, the well-known port for POP3S, i.e, an encrypted POP3 connection using Transport Layer Security (TLS) or Secure Sockets Layer (SSL). I then checked the box for "This server requires an encrypted connection (SSL)".

    Outlook 2010 - POP3S encrypted connection

  7. After clicking on OK, I clicked on Test Account Settings. I then saw "Congratulations! All tests comleted successfully."

    Test Account Settings
completed successfully

  8. I then clicked on the Next button. The test was repeated, but then I was taken to a window where I could click on Finish.

    Outlook 2010 - Congratulations

  9. I then clicked on Close to close the Account Settings window.


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