How to reply emails with original content preserved using smtplib and imaplib

Question

I want to use Python's smtplib and imaplib to implement reading emails (such as Gmail, Exchange) and replying to the original email. What I want to do is read an email, add a new message to the original email while preserving the original message, just like using Gmail's reply function. The new message might be in HTML format. I've searched through smtplib and imaplib documentation but haven't found any similar reply functionality. I can send or reply to emails, but I can only send new messages without being able to retain the original email's content.