Dear Omolade,
I'm not sure if you were looking for a response form me, but what I would like to know is if it's possible to add a memo only visible to myself to a message received from somebody else, that I need to reply to later.
Basically the same as when you receive a letter which has all sorts of information you are forming some ideas about on how to respond to that. And those ideas you jot down on a physical yellow post-it note.
So that when you have the time to respond to the letter carefully, you will remember the first thoughts that came up when reading the letter for the first time, which are often the most accurate and important ones (in my case at least).
So this personal reminder should NOT be sent along with the reply you ultimately come up with. Same way you would not stick the physical post-it note containing personal/initial/ thoughts in wording not suitable for a formal letter, on a physical letter of reply.
Hope this helps,
Best, PJ