My Yahoo IMAP stopped working on 1/14, the started working for a few hours, catching up on all the missing emails. Now its not working again. iOS devices are fine. The problem appears to be widespread.
https://discussions.apple.com/thread/6776426
https://discussions.apple.com/message/27488890#27488890
https://discussions.apple.com/message/27490557#27490557
https://discussions.apple.com/thread/6776426
https://discussions.apple.com/message/27488890#27488890
https://discussions.apple.com/message/27490557#27490557