A user received an email, which was responded. Where the response will be sent?

$1.$ To the email address in the from header, in any case

$2.$ To the email address in the from header, which overrides the return$-$path header.

$3.$ To the email address in the reply$-$ To header, if it existed, or the from header

$4.$ To the email address in the reply$-$ To header, which is mandatory.