Did you actually read the discussion that I linked to? https://mail.python.org/pipermail/mailman-users/2014-August/077673.html To quote from it:

...

[...] Outlook sends Reply All to the original sender and anyone else that was Cc'd, but not the list.

Is there a fix? Which other variables affect this?

No, there is no fix. Both behaviors are arguably RFC conforming (the RFC doesn't specify "Reply to All" behavior of MUAs, so the MUAs can do whatever they want), and Outlook's is arguably more conformant to the spirit of the RFC, since according to the RFC the configuration

From: list at example.com Reply-To: person at example.org

has the semantics "the entity using the mailbox 'list at example.com' requests that you address mail intended for the entity to the mailbox 'person at example.org'".

-- Nathan -----Original Message----- From: J. Hellenthal [mailto:jhellenthal at dataix.net] Sent: Wednesday, July 20, 2022 7:29 AM To: Nathan Anderson Cc: Voice Ops Subject: Re: [VoiceOps] Mailman 'Reply-To' munging (was: Re: 9-8-8 dialing [...]) "Reply-To in different ways" ? It means one thing... am I missing a mail client that does not follow this ?

On Jul 20, 2022, at 06:24, Nathan Anderson via VoiceOps <voiceops at voiceops.org> wrote:

"Reply-To" in different ways

-- J. Hellenthal The fact that there's a highway to Hell but only a stairway to Heaven says a lot about anticipated traffic volume.