I had a user with this exact same issue, but was able to resolve it.  Here is what I did:

1. Disabled caching in Outlook (resolved the issue but Outlook took a performance hit)

2. Updated Outlook to most recent version through Office updater.

3. Enabled caching in Outlook

While performing this, I archived the users content as they had over 26GB of emails.

Once this was all completed, the user was able to edit Quickbooks emails.  Not only that when the email was sent, it retained those changes.