I have an issue accessing my Yahoo Inbox using IMAP with Mail 8.1 on my MacBook Pro running Yosemite 10.10.1. The Connection Doctor confirms connection and login for both IMAP and SMTP. When I select Get Account Info at the bottom of the left column in the Inbox pane, the Quota Limits tab shows the number of messages in several mail folders I have on the server, but shows the Inbox with 0 messages, which is not correct. When I select Get New Mail from the Mailbox tab, the Yahoo mailbox is grayed out. On one attempt when it was not grayed out, it returned the message "The server returned the error: the attempt to read data from the server imap.mail.yahoo.com failed".
The following solutions found online have been tried without success:
--take account offline then Get All New Mail
--remove and reinstall the Yahoo internet account in System Preferences
--remove the Yahoo internet account, then re-add as an "Other" mail account
--delete com.apple.mail.plist files in the Library, then restore by restarting the program
--delete com.apple.mail folder in the Library, then restore by restarting the program
--reduce password characters to 8, alphanumeric
As a final attempt, I removed Mac OS X from the "Manage apps and website connections" section in my Yahoo account profile. I removed and restored the Yahoo account in Mail, but this did not restore Mac OS X in my Yahoo account.
IMAP access worked fine until just a few days ago. Access through Web browser and my iOS devices has fortunately been unaffected. What broke?
MacBook Pro with Retina display, OS X Yosemite (10.10.1)