Design flaw permits storing of incoming emails in the "sent email" storage bins.
The Thunderbird, including its Portable version, allows the user to move incoming emails in the bins where emails already sent out are stored, and the other way around. This mixing of the incoming emails with out-went emails is a problem when searching for archived emails later. The design engineer should implement a module to prohibit this kind of operation. In other words, if a user drags an incoming email to the out-went email storage bins, the system should give an error message: "You Cannot store incoming emails in the out-went email bins!"