You could even skip the custom system prompt entirely and just have it analyze a randomized but statistically-significant portion of the corpus of your outgoing emails and their style, and have it replicate that in drafts.
You wouldn't even need a UI for this! You could sell a service that you simply authenticated to your inbox and it could do all this from the backend.
It would likely end up being close enough to the mark that the uncanny valley might get skipped and you would mostly just be approving emails after reviewing them.
Similar to reviewing AI-generated code.
The question is, is this what we want? I've already caught myself asking ChatGPT to counterargue as me (but with less inflammatory wording) and it's done an excellent job which I've then (more or less) copy-pasted into social-media responses. That's just one step away from having them automatically appear, just waiting for my approval to post.
Is AI just turning everyone into a "work reviewer" instead of a "work doer"?