I just found http://www.indev.ca/MailTags.html but I will not pay 30 bucks just for adding some notes to my mails.
Are there some free extensions or alternative mail apps that support adding notes to emails?
That would be awesome. I searched a lot but didn't find a good alternative.
The point is: I want to add a simple comment/note on my mail so I directly can identify the state (e.g. 'waiting for response with data')