Autoflagging in Outlook 2007

I have a request from one of my users.

She says that some mails arrive, and are automatically flagged for some reason.
We`ve examined the rules, and there isn`t one there that would auto-flag certain mails (actually, all the rules do is move from certain senders to specified folders).

So now i wonder : Is there some setting that a sender can use to autoflag these mails? And, how can you prevent this?