Annotated emails in OS X Is there some email client out there, which is well integrated into OS X, that has the ability to annotate emails? In particular, I would like to add a note, which will be linked to a specific message.

Any ideas?
Have you looked at MailTags? I believe that in addition to tagging email messages you can also attach notes to messages. It is an add-on to Mail.app: http://indev.ca/MailTags.html. They also have a preview for Lion: http://indev.ca/MailTags3.html.