Cannot get email on cellpheone, Apple I 4

Hello, For some time I have been unable to send or receive email on my cellphone, although I can on my computer. It appears there is a lock after my user key? I have checked with my internet provider several times. Because I can access other online sites on my cellphone, Comcast determined that the problem was on the yahoo end. Anything you can do to solve this problem will greatly appreciated.
Sincerely, William <REMOVED BY MOD>