I don't know what sort of mail environment you have but at my office we're running Exchange and almost everyone has a company BlackBerry. It is possible for my users to accidentally delete an e-mail from their BB without having read it -- if their BB is set to 'Delete Confirmation = No'. I'm just saying that sometimes the issue isn't a matter of honesty.

However, whenever I run into people such as you describe, my co-worker and I have an agreement to copy each other on the e-mail (or reply). This does two things for us: It let's the recipient know that someone else knows about the (non-confidential) matter; and it keeps my co-worker and I in sync on the issue. This has the added benefit of keeping both of us from working on the same requests/issues.