Note: EverMail is no longer for sale. Its functionality is now included in Feigeist's MailButler, whose link can be found int he following paragraph.
EverMail--which is now a part of MailButler, the complete assistant for Apple Mail--allows you to easily convert emails in Apple's Mail application to be converted into notes in Evernote. This conversion includes formatting, file attachments and all other information regarding that particular email. Notes created by EverMail contain a direct reference to the source email that it was created from. This allows to easily find the email correspondence from within Evernote. The same works in the other direction: Notes are directly accessible from your list of emails in Mail.