While Mac OS X Mail does not allow you to go ahead and edit emails as you view them, you can, in a bit roundabout a manner, create a copy, edit the copy, and replace the original with your custom copy. It'll be easier than it may read.
Edit Received Email in Place with Mac OS X Mail
To edit a message in a Mac OS X Mail folder:
- Drag and drop the desired message out of Mac OS X Mail onto the Desktop.
- Ctrl-click on the message copy on the Desktop.
- Select Open With | TextEdit from the menu.
- Make the desired changes to the message source.
- To change a message's subject, look for "Subject:" at the beginning of a line starting from the top.
- Close TextEdit saving the document.
- Ctrl-click on the message on the Desktop again.
- Select Open With | Mail from the menu this time.
- Now select Message | Copy To followed by the message's original folder from the menu in Mac OS X Mail.
- Close the message window.
- Delete the message copy from your Desktop.
- Optionally, delete the original message in Mac OS X Mail.