[u801e]

Is there anything in the email headers that shows the authenticated user? My preference would be that email providers rewrite the From: header to match the authenticated user and that the sender uses the Reply-To: header to direct replies to a destination of their choice.

[distances]

Not good enough, as many email clients show both of these fields. The whole idea why people use this is to send and receive with one address only, even if you've authenticated with another.

[u801e]

I believe most email clients will default to using the Reply-To: header to determine the value of the To: header in the reply, so if both headers are specified and even if the email client displays both of them, then the recipient would have a chance to determine whether the email is authentic or not.

[forgottenpass]

Fastmail includes a header - decodable only by Fastmail - that can identify the sender account. And IP address if sent through SMTP.