I've never heard of this. However, in Outlook Express, it can automatically put anyone you reply to in the address book. If you sent a "Foward All" reply, it would add everyone that's in the address line. The path is >Tools>Options>Send, uncheck the box that says "Automatically add..........."
Not knowing what email program you're dealing with, it impossible to say if it an option to add addresses just beacause you RECEIVED it. But if it is, I would think it could be turned off by a similar path.

