Tips and Tricks for AppleMail
Most likely you have been in the same situation: the mail quote is almost full and some nice collegue sends you the 50 MB attachment you never wanted to have. Or you want to keep some important mails but the subject is as bland as unsalted rice. So editing capabilities in a mail are sometimes life saver. But Mail sometimes makes life rather complicated.
Normally, Mail won't let you edit incoming mails. But there is a workaround: drag the mail to the drafts folder. Do a double-click to edit the message. After saving the message drag is back to the Inbox.
If you don't have a Draft folder then you first need to create a temporary draft message.
Drawback: The date of the message will change to the date the message was saved.
The screenshot shows a mail in the Draft folder. It's just been edited and the received time has changed.
Source: Bower Web Solutions