smail vs. sendmail

[Bill Wisner]

Two points:

Reply-To: and In-Reply-To: lines are a function of the user agent, not the
delivery agent. Specifically, it is the user agent that must insert an
In-Reply-To: header when a message is replied to, and it is the user agent
that must notice a Reply-To: line and use it to construct an address to
reply to.

Smail3 is RFC822 conformant and munges Cc: lines on outbound mail correctly.
A major goal of the authors is to make it into a complete plug-in replacement
for sendmail, so when it is released you should have little trouble.