Yes, it's on Yahoo's side, and it needs to be fixed there.
It's so nice of Yahoo to not provide any support, only a "community forum where you questions will be answered by other users like you" (maybe I should stop support too... oh wait, I guess only multibillion corporations with celebrity CEOs can do that...?)
Please see below:
http://www.aqua-mail.com/?p=1981Apple Mail is also affected, looks like same issue (look at the number of posts and their times / dates):
https://answers.yahoo.com/question/index?qid=20150114021156AA4PjIXThere is no "deficiency" here -- the IMAP command used in Aqua is perfectly valid, has worked with Yahoo from 2010 (!!!) until now, and continues to work with all other IMAP mail services known to man or woman.
I have not had a single report of this issue until yesterday when I got 4 or 5 messages, and about a dozen today. Yahoo must be doing a "staged" (gradual) update.
As for K9 -- IMAP is a rich protocol, and has many command and many ways to use them.
If it so happens that K9 doesn't use the command that Yahoo broke -- well, good for them -- but there is no guarantee that Yahoo (or some other mail service) won't break something used by K9, Kaiten, Boxer, or any other mail app, mobile or desktop. And then the tables will turn.
And to reiterate -- Apple Mail is also affected, apparently they are using same IMAP command to ask the server for the list of unread messages in a folder.
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Now that being said, I made this test build where I replaced one perfectly valid IMAP command with another perfectly valid IMAP command. Maybe Yahoo's server will be able to process that other one?
http://www.aqua-mail.com/download/AquaMail-market-1.5.1.12.10.apk
You can update in-place without a full reinstall.Update: does not help, details below, two messages down.
Another update (January 15): looks like Yahoo has recognized the issue and started to implement a fix. Fingers crossed