It's a common mistake to think that a sender should insert a Return-Path header.
Receivers use the envelope sender in MAIL FROM command to return asynchronous/remote bounces. The Return-Path header is inserted by receivers to preserve the envelope sender when it is delivered via other means than SMTP. See section 4.4 in RFC 5321, the "SMTP standard":
When the delivery SMTP server makes the "final delivery" of a
message, it inserts a return-path line at the beginning of the mail
data. This use of return-path is required; mail systems MUST support
it. The return-path line preserves the information in the <reverse-
path> from the MAIL command. Here, final delivery means the message
has left the SMTP environment. Normally, this would mean it had been
delivered to the destination user or an associated mail drop, but in
some cases it may be further processed and transmitted by another
mail system.
