EverMail 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.
