hi there im having trouble on adding my yahoo email account on to my iphone which i previously had it.

after deleting m y yaphoo email account from my iphone I' m unable to add it back on to my iphone. everytime I try to add my yahoo email account using setting>> mail >> add an email address>> select the type of email address ie yahoo>> email address (rubl000@yahoo.ca)>> password. the error messages prompts as "server Unabailable". Will you please help me setup adding this email account on to my iphone? Please and thank you!