Outlook 2010 0x800CCC91 error - plaintext authentication
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.
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.'"
To correct the problem, I changed the Outlook settings for his account
to use encrypted authentication by the following steps:
I clicked on File on the menu bar at the top of the Outlook window.
I clicked on Account Settings.
I then clicked on his email account under "Name" to select it and then clicked
At the Change Account window, I clicked on More Settings
I then clicked on the Advanced tab in the Internet E-mail
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)".
After clicking on OK, I clicked on Test Account Settings.
I then saw "Congratulations! All tests comleted successfully."
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.
I then clicked on Close to close the Account Settings