In reference to this techrepublic article Five ways to add a note to an Outlook email message I’d like to make a remark.
I don’t like any of these methods for larger notes. The lack of implicit connection between Outlook emails and OneNote notes is problematic as it requires manual housekeeping.
My preferred method is to create a note in Outlook, then attach it to the email (in edit mode). This means the note is literally attached to the email (which is what this article is trying to achieve), and they are intrinsically connected.
I also using the flag for its intended purpose – a short message with an optional reminder. Once the email is resolved, then I can choose to delete the attached note. But flagging is also great as it causes the email to appear in Microsoft To Do. But I wouldn’t use the flag for notes. The flag should be made up of text that completes the sentence “Flag to…” per the custom flag dialogue.