I use Mac Mail on my iMac to manage my Yahoo! mail account. Suddenly, yesterday, I wasn't getting new email messages and also would get the error message The message "Test" could not be moved to the mailbox "Yahoo!" with smaller explanation The IMAP command "APPEND" (to Sent) failed... when I composed a new email. (New emails do send successfully, however.)

I used Connection Doctor -- successful connections to Yahoo! account every time, but no new mail, quit Mail and restarted it multiple times, restarted computer several times. Also, removed Yahoo! account and then added it back. Tried rebuilding Mailbox several times. Sometimes shows that it is receiving x number of messages, but they don't show up. At one point, ALL of my emails disappeared in Mail. Somehow they showed back up late yesterday, with the new messages, but I seem to be back at point zero with same problem.

I can view email on my iPhone and in my internet browser but really prefer using Mail to compose new emails and manage all of it.