The “Reply-To:” header is a standard and legitimate header field in emails.
emClient has no way of populating that field on a per-message basis. Setting up Alias accounts is a different thing altogether. A message sent from an account with a different “reply to” field is perfectly legitimate and simply redirects replies.
The suggestion is to include a “Reply to” or “From” field in a compose form (it could be optional like the CC/BCC button) which will populate that header field. Postbox, Outlook, Thunderbird, all have this facility.