On your Exchange Server, open AD Users & Computers, it should be under Start/All Programs/Microsoft Exchange.
Find the relevant mailbox used by your application and open it's properties page.
Select the "Exchange Features" tab.
On the tab you should see a "Protocols" section (example).
If you click on IMAP4 and then Properties, you should be able to force plain text there.
если у тебя виндоз это вообще видал одной нопкой делается