You want to add notes to your mails or conversations in order to remember in the future for example which was the conclusion of the conversation, which pending tasks i have related to it or why i starred this mail?
Then this will be useful for you.
How it works:
1. Open a mail/conversation in gmail.
2. Click on the edit notes button on the top right buttons bar (the gray pencil).
3. Enter the notes/commentaries you want for the current selected mail.
4. The notes text is automatically saved when you modify it.
5. You can close the notes popup with a click in the edit button.
To know:
. The notes are stored in the user computer.
. It is fully integrated in the gmail tab.