This problem happens for IMAP accounts, and seems to happen when people have Mail store draft messages on the server. Any changes to the local drafts folder will be synchronized to the server so you can access your drafts from alternate e-mail clients such as Webmail, another computer, or a mobile device. If you use IMAP e-mail accounts, Mail may automatically enable the option to save a copy of these draft messages in a corresponding drafts folder on the IMAP server. If you do purposefully save them, they will be removed either upon being sent, or otherwise only if you remove them manually. If you do not save them directly they will stay there until the message is either sent, or discarded without being saved. The way Mail handles draft messages is that a local draft folder is created on your computer where these messages are automatically saved when you start composing. While not an intrusive issue, and definitely one that can be fixed by deleting the messages in this mailbox, some people have found the drafts are continually and permanently saved every time they compose new messages. One relatively common problem people have with Mail is that the program will seem to collect messages in its Drafts mailbox.