Per-mailbox IMAP settings in Exchange 2003 apply successfully but revert to server default

The title says most of it.

I have a Spiceworks mailbox that connects to our Exchange Server 2003 box via IMAP for receiving help desk issues. But for complicated reasons, I want it to receive those emails in text-only format.

So, I discovered that you can just go to:

• Exchange System Manager
• Administrative Groups
• First Administrative Group
• First Storage Group
• Mailbox Store
• Mailboxes
• Right-click the mailbox, Configure Exchange Features
• Edit the properties for IMAP
• Set that mailbox to only receive message bodies as plain text.

I click OK, then Next, it reports success, and I assume I'm done. But then when I go right back to where I was, I see that "Use protocol defaults" is still checked. Anyone have a clue why this would be?

Some other details: I'm logged in as Administrator when I do this. I can't change this setting for the entire IMAP virtual server because some regular users use it. I only have one IP address to play with, which means I can't create another IMAP virtual server. Any suggestions or ideas are greatly appreciated!

EDIT: this question was resolved by joeqwerty in the comments. If possible, I'd like the mods to mark this question as answered.