Yahoo have updated there email settings so you can't access them without using "SSL"(security settings in a nutshell.)
The link below has the settings, which if you can on your iphone/ipad go to
SETTINGS
MAIL, CONTACTS,CALENDER
click YAHOO in accounts
ACCOUNT INFO
ADVANCED
you should find the settings there (ports and enable/ use SSL etc)
http://help.yahoo.com/kb/index?y=PROD&page=content&id=SLN3697&actp=search&viewlocale=en_US&searchid=1386851985635&locale=en_US&y=
However I deleted my old Yahoo account on my iphone as the options I needed to chage for "SSL" where all greyed out.
So to set a new one up I have to do this....
SETTINGS
MAIL, CONTACTS,CALENDER
under accounts click ADD ACCOUNT
select OTHER as YAHOO didn't work for me with the new security settings
under mail click ADD MAIL ACCOUNT
in the new account section fill in...
NAME
ADDRESS...your email address
PASSWORD
DESCRIPTION...I just call it YAHOO! as I have several emails set up on y phone
click NEXT
click IMAP
under incoming mail server fill in...
HOST NAME..... imap.mail.yahoo.com
USERNAME.... your email address
PASSWORD...your password
under outgoing mail server fill in...
HOST NAME..... smtp.mail.yahoo.com
USERNAME.... your email address
PASSWORD...your password
click NEXT and PREY
