Oh my goodness!! I have been trying to set up my free yahoo mail for 3 days now and ONE simple thing from all of these posts helped me out! I have a free account, my password is small and I set up with IMAP and I was still receiving the caution triangle symbol in the actual apple mail. All is took was for me to enter into apple mail, click on- mail- on the menu bar in the upper left hand corner, drop down and click on - preferences, click on the yahoo mail icon, find -Outgoing Mail Server (SMTP) and open the box. At the bottom click- edit SMTP server list and in the Server Name box it should read- smtp.mail.yahoo.com - Ok, all you have to do is add a : to the end of it!! That's it, a colon!! So now it should read- smtp.mail.yahoo.com: This is just for those of you who have set up everything proper with IMAP with the port 993 and its still not working. I hope this helps!
On my iPhone 5 (iOS 6.0.1), I got Yahoo mail working via POP using the info on this page:
For both incoming and outgoing mail, use your full Yahoo e-mail address as your 'User Name' and put your password in both 'Password' fields. Turn SSL on, use password authentication, and server port 465.
I know this original post is from a while ago, but people are STILL having trouble getting Yahoo! mail to work on the Apple Mail application (at least I was), so I wanted to share my solution since the original post did not work for me (this is for my Macbook, however the same principle is used for iOs deviced)
On your Mac, go to settings > internet accounts
Click the + sign to create a new account
Choose the Yahoo! option
Enter your name, user name (make sure to put @yahoo.com after the user name)
For PASSWORD, do NOT enter your usual Yahoo! password (this is where I was able to fix my problem):
Open up your browser and go to Yahoo! and sign in as you normally would; on the upper right-hand corner hover over your user name and click "Account Info"
Click the "Account Security" tab
Scroll down to "App Passwords"
Under "Nickname your app" you can put in something like "Macbook Apple Mail"
Click "Generate Password"
You'll get a random password - highlight and copy it (command-C)
Now paste this password into the Password spot under the Yahoo! Mail setup in your Macbook settings
When it tells you to select the apps you would like to use for this email address (Mail, Contacts, Calendars, Notes, etc...), make sure Mail is check-marked.
Now the account should show up in your Mail application!
THIS should work for you. At least it did for me. I hope this helps someone!
I'm really sorry - I don't know why it wouldn't be there. I do believe this will only work if you have two-step verification turned on. Here's a picture of what my security page looks like - you can see two-step verification is turned on, and the APP PASSWORDS is below that. Best of luck - sorry I can't be of more help!