I've lost a number of archived emails going back to at least August of this year. Pretty much any emails I've moved to folders on my iPad since October simply vanish or become unreadable on the web site (gives read error). This doesn't happen if I use the mail.yahoo.com web site or another IMAP program such as Mozilla Thunderbird.
I've complained to Yahoo (as I'm a Mail Plus user) and they claim they're looking into it. They also keep referring me to this
"hidden" support document claiming the emails were lost do to a hardware failure. The problem with that document is that it says the problem has been fixed, but the problem is still happening. Not only that, but the date on the document keeps changing. When I first reported the problem that document had a last updated date of October 20, 2011. Now it's December 1, 2011.
If the problem was fixed in October and again in December why is it still happening and only when moving emails on my iPad or iPhone?
In the mean time I've started archiving my mail on my PC by using IMAP to copy the emails locally. I've even managed to restore a few lost emails by copying them back to Yahoo using IMAP, but there's still a number that have been lost for good which really pisses me off considering how Yahoo still won't really even acknowledge the problem.
I've also complained to Apple who ignored me.