When you flag an item, you are free to choose the text for the flag. By default the text is set to “Follow up” and there is a list to choose some from as well, but you can type any text you want.
In order to type the text, right click on the Flag column in Outlook or press CTRL+SHIFT+G and choose “Custom…”.
Type your custom text for a flag
You can set the Start date and Due date to None.
Modify the subject
If your notes are only short, then you can double click the message to open it in its own window, and directly type your note in the Subject field.
Type in the message body
If your notes are a bit longer, then you can also type it directly into the message body. In order to do this, you must double click the message to open it in its own window. Then choose;
Other Actions-> Edit Message
Once in edit mode, you can freely type in the message. When closing the item, make sure you save the changes to save your note.
Attach a Note
If you want to add a proper note, then you’ll have to write it in the Notes section of Outlook first. After this open the message and place it in edit mode as indicated above. Now you can drag and drop your note into the message and it will show as an attachment. You can delete the note from the Notes folder now as it will be saved with the message.