Starting a few hours ago, I've been having issues downloading mail to my iPhone and iPad using the built-in Mail app. From what I can tell, there are intermittent issues with the IMAP servers.
Changes to flags and unread status will propagate as will deletions of mail messages, but messages themselves won't download most of the time. The Mail app will say "download 1 or x messages" and the progress bar will appear, but then it disappears and no mail is downloaded.
It seems to depend on which of the apple.imap.mail.yahoo.com servers my iPhone/iPad connects to. Since these are on round robin, if the iPhone/iPad connects to a problem server, mail won't download. If it connects to a working one, it will work. This has the effect of mail sometimes not downloading on either my iPhone or iPad or sometimes downloading on one, but not the other.
I'm currently tracking the connections my devices make and currently I'm seeing the following servers working or not working (though this has changed).
Working:
98.139.212.64 - vcs-u3-star-i-myc.mail.vip.bf1.yahoo.com
98.139.172.225 - vcs-u2-star-i-myc.mail.vip.bf1.yahoo.com
Broken:
98.139.244.126 - vcs-star-i-myc-mail.vip.vf1.yahoo.com
98.138.24.48 - vcs-u2-star-i-myc.mail.vip.ne1.yahoo.com
98.138.215.4 - vcs-star-im-yc.mail.vip.ne1.yahoo.com
From what I can tell, the IMAP message id's are being reported wrong on the broken servers so the download fails. How can I get Yahoo to fix this?
Added (1). It started working more often than not prior to your "fix". After your fix, it simply re-downloaded all the old mail, but not much else changed.
For the most part now, instead of failing 90% of the time, it is working 90% of the time. It still has issues occasionally though.
Added (2). It turns out you made things a lot worse. I use Mozilla Thunderbird to read my Yahoo Mail via IMAP and now it's downloading all my messages again, but because the problem is still there it's spitting out a lot error messages for a lot of them. The error is:
"The current operation on 'folder' did not succeed. The mail server for account myyahooid@yahoo.com responsed: [UNAVAILABLE] UID FETCH Server error while fetching messages."
Where 'folder' is the folder it's trying to download (it's failing on all of them) and myyahooid is my Yahoo! Id.
It's spit out over 50 of these errors in the last 13 minutes (averaging about 4 a minute). This is because the "fix" you did, didn't actually fix the problem with the servers, it just reset my IMAP account. Now my PC is downloading several thousand emails and failing on about 1% of them.