Honestly I'm about ready to give up on trying to use Outlook.com.   I can't believe it can't do a simple thing like connect to a Yahoo Plus Email Account that Outlook Express 15 years ago could and Live Mail can.  I've read through the other helps and it appears to be a common problem that's been going on for almost a year.

1.) The error I am getting is: We couldn't sign in to your POP3 server. Please make sure your username and password are correct.

2.) The email address (username) does have an underscore in it but I have also tried my other account that does not.

3.) The password only has letters and numbers in it and is between 8 and 12 characters.

4.) My yahoo account is a plus account

5.) As I said I can connect to the account just fine with Windows Live Mail and actually at first tried importing the account and messages from Windows Live Mail.

6.) I have tried both the plus.pop.mail.yahoo.com and pop.mail.yahoo.com for the pop server.

7.) For the username I have tried both the name and the *** Email address is removed for privacy ***

8.) Yes I'm sure the password is correct.

Basic Settings in Outlook.com

Name = my name

email = *** Email address is removed for privacy ***

POP3

Server address = plus.pop.mail.yahoo.com

Port=995

Require a secure connection is checked

Leave messages on server is checked

Username= my_name

Password = 8 to 12 letters

SMTP

Send email using your provider's server (recipient will not see your Outlook address)

Server Address = smtp.mail.yahoo.com

Port = 465

Requires a secure connection (SSL/TLS) is checked

Use the same username and password to send and receive email is checked