Sure. How’s that relevant?
That someone might use IMAP is no worse than someone using a photocopier.
Sure it is, and your own comment above about gmail to gmail being fairly secure demonstrated that. Using a photocopier is intentional, and everyone knows what a photocopier is. Most people don’t know what IMAP is, and an email sender does not know if the recipient uses IMAP.
And this is still irrelevant to sending email to the wrong recipient, so I don’t know why you’re stuck on infra security.
No matter what format you hand a recipient a document in, they can always make a photocopy and pass it on.