Can't access Yahoo account on iphone- says 'IMAP server"apple.imap.mail.yahoo.com" isn't working'
The incoming IMAP server you are getting ("IMAP.apple.mail.yahoo.com") is not a real Yahoo IMAP server. Like @Buscar said, deleting and re-adding should resolve the problem. However, it seems like the settings are getting recreating incorrectly.

What I'd do, after re-adding the email, is manually fix the settings to the right servers. To fix them, go to Settings > Mail, Contacts &amp; Calendars > (select Yahoo account) > Account. You should have the option to edit the IMAP server in the Advanced section.

If it doesn't allow you to change it, add the account manually by going to Settings > Mail, Contacts, Calendars > Add Account > Other. You can fill in the information found here: https://help.yahoo.com/kb/mobile-mail/imap-server-settings-sln4075.html