After installing Rollup 1 for Exchange 2007 SP3 IMAP is now serving most text/plain parts as text/html instead. I had some issue with a simple mail import tool that now imported all mail with html codes.
Before the rollup 1 they were previously multipart/alternative messages with both text/plain and text/html subparts.
You can set in through in the GUI or using Powershell to set the value to 0 text only
2:HTML and Alternative Text
3:Enriched Text Only
4:Enriched Text and Alternative Text
5:Best Body Format
Exchange Management Console:
In the console tree, expand Server Configuration, and then click Client Access.
In the work pane, click the POP3 and IMAP4 tab.
In the work pane, select IMAP4 and then, in the result pane, click Properties.
On the IMAP4 Properties page, click the Retrieval Settings tab.
In the Message Retrieval pane, select a message format from the drop-down list under Message MIME format. I choies for 0
Click Apply, and then click OK to save your changes.
Set-CASMailbox -Identity yourcasserverhere -PopMessagesRetrievalMimeFormat 0