OK, I think I figured this out, for my specific needs.
Recapping, I wanted to achieve just this one feature of "Discussion Forum/List" management, (which I used to have in majordomo on my old email server):
Recipients of messages to a distribution list click "Reply" in their email client, and their reply goes to the list, not just the original sender.
(Training the recipients to click "Reply-all" has proven futile. Human behavior and such, meh.)
The email clients I tested for this solution are Eudora 4.3.2, (older than dirt, I know), Outlook Express 6.00, MS Office Outlook 2003, and Zimbra UI 4.5.10.
To make it work, I started with the suggestion in this thread from Freddie (thanks very much!).
I had to add another test to look for angle-bracketed "To:" addresses so that replies to the first reply also go to the distribution list.
Apparently, the angle-brackets, < >, appear around the To: address only after the first recipient replies to the original message, I don't know why.
I added rules to header_checks in postfix as follows:
On my Zimbra 4.5.10 server, logged in as user zimbra, in the directory, /opt/zimbra/conf , I made the file, postfix_header_checks.in, writable by zimbra with:
# chmod u+w postfix_header_checks.in
I edited that file to append these lines:
The additional angle-bracket entries, above, allow the reply-to-list feature to continue to work after the first recipient's reply, such that subsequent replies also went to the list, (at least they did in my testing.)
if /^To: firstname.lastname@example.org/
/^To: email@example.com/ PREPEND Reply-To: firstname.lastname@example.org
if /^To: <email@example.com>/
/^To: <firstname.lastname@example.org>/ PREPEND Reply-To: email@example.com
if /^To: firstname.lastname@example.org/
/^To: email@example.com/ PREPEND Reply-To: firstname.lastname@example.org
if /^To: <email@example.com>/
/^To: <firstname.lastname@example.org>/ PREPEND Reply-To: email@example.com
Then I did a
# postfix reload
and ignored these warnings:
The warnings could probably be suppressed if I knew the right syntax to put into postfix_header_checks.in, but this solution works, for me, for now.
postmap: warning: /opt/zimbra/conf/postfix_header_checks, line 6: record is in "key: value" format; is this an alias file?
postmap: warning: /opt/zimbra/conf/postfix_header_checks, line 7: expected format: key whitespace value
postmap: warning: /opt/zimbra/conf/postfix_header_checks.db: duplicate entry: "if"
By editing the file, postfix_header_checks.in, my changes should survive a server reboot, but probably not a Zimbra upgrade.
If I get some feedback of interest, I'll post or append-to a pertinent Zimbra Wiki article.