I can't pin this bug to anything specific so I'll try to give as much info as possible. This bug started about 3 or 4 months ago for me. But around that time I did enable the "Today" inbox and that is what I use 90% of the time now. The problem is I'll swipe left to send a handful of e-mails to the trash then leave the app. Next time I check e-mail most of them are back. I have to swipe left to delete them all over again. Usually after two or three deletes they stay gone. What gives?
I'm running iOS 9.3.4 & OS X El Capitan 10.11.6.
UPDATE: I downloaded Airmail. In Airmail I added my @icloud account and one of my @gmail accounts. @icloud.com e-mails still won't stay deleted, they keep reappearing (not all, just random ones). @gmail seems to be fine. So it doesn't appear to be the app that I'm using. Is this an issue with @icloud email?
UPDATE: I tried rebuilding the mailbox twice using the default Mac Mail app. No joy. After it was rebuilt I selected about 10 emails to delete, press delete, it says in the bottom left "Moving Messages", stays that way for 2-10 minutes, then finally goes away. I switch to my gmail inbox then switch right back to the icloud inbox and they are back again. I've repeated this many times. When it says "Moving Messages" in the bottom left it seems to take a very long time. Very frustrating. Sometimes I've seen an error message popup saying it couldn't delete because mailbox was busy (something like that).
I also tried accessing mail via www.icloud.com, after selecting a handful then deleting, and waiting a long while, it would throw a message saying it couldn't delete them. I then tried deleting 1 at a time via the icloud website and they delete but it thinks for a long time before it actually disappears. So it seems like deleting is veeeeery slow and works 1 at a time via icloud website, but multiples don't delete/timeout or something.
Any help would be appreciated! I couldn't find any solutions on the Apple forums.
UPDATE: I changed my icloud password just in case it was something malicious. Didn't help. I did find these:
https://discussions.apple.com/thread/6560166?start=0&tstart=0
https://discussions.apple.com/thread/7592837?start=0&tstart=0
Do you think icloud is having issues and it's still running slow and that is what is causing my delete issues?
Best Answer
This also occurs in Outlook for iOS.
It's an iOS threading (GCD) issue of one of two kinds, either:
GCD places low-priority on threads to which calls for "delete" are dispatched, and defers to any and all other operations on, like, any other thread; or,
GCD stops waiting for a condition to be met on the mail server side, suspends or kills the thread in favor of others, and then fails to resume or reinstate the thread at a later time.
It could also be improper implementation of GCD in a networking component of iOS, which happens easily enough for programmers with less than 20 years of experience with multi-threading behind them.