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.