Maybe I should make it more clear: you send a question to sendmail.org,
you can get an answer from esmtp.org, some university, some Unix
vendor, some other domain...
Hence your original mail must already include a "token" which allows
the reply to go through without depending on the sender address
(domain). The simplest way would be to allow mail with
Subject: Re: your original subject
through... (hence "your original subject" should be a bit more
descriptive than "Hi" or "Help"...)
Oh, I got you. And yes, you have a good point.