This really needs to be brought up with Apple -- if this server was "forgotten" in a software update, it can be updated and stop working at any time.
There are two major ways to retrieve messages with IMAP:
1 - Ask for whole message at once (possibly up to a limit, but still, the whole message) -- and break up into text, attachments etc on the client. K9 and Android Email work this way, Outlook does too I believe.
2 - Ask the server to parse a message into parts, and then download specific parts (text content, an attachment). Aqua works this way.
I'm guessing the corruption happens when a mail app asks for a specific attachment (option 2) and not the whole message (option 1).
Needless to say, option 2 used by Aqua works just fine with dozens if not hundreds of mail servers / services from AOL to Zoho, and until approx. December 2014, worked fine with Apple's mail services too.
