It's a Mailman mailing list. C'mon, man, they're not that old-school yet.
I just facepalmed that
@SixPapaCharlie doesn't know how a mailing list works. Especially Mailman. LOL.
Sounds like it's in "Digest" mode by default at GG.
There's usually a URL at the bottom of each message with a web page where you can set settings for how you want it to work.
Originally mailing lists were just like the ones at most offices. Send a message to the list address, everyone gets a copy.
You wanted to reply to one, you did and the mailing list would leave the Subject line intact so a reader could sort by Subject and see all the replies for that topic together in their mail program. (Or if they were sadists they'd leave that folder sorted by date/time and jump around reading only the Subject line they were interested in.)
Later as traffic went up, people didn't want to bother to take the time to build filters to put say, 100 messages an hour, into a folder marked "Scan this mailing list crap later..." so they asked the mailing list people to send them one big email with summaries of all topics being discussed.
Problem with that always was and is: The subject line has been mangled into something that says it's a summary. And people respond to the list address without the original Subject line of the thread they were talking about.
No worries, just reply anyway and everyone gets a copy. They'll figure it out. If you're polite, list etiquette says you snip all but what you're quoting, never top post above the quoted material, and edit the Subject line to match the discussion thread you're continuing. (Removing any stupid "Re:" your mail program "helpfully" adds.)
You can go to the URL at the bottom and set up a login (your email address) and password to change the list behavior. I always did. I switched off Digest mode and made a filter to put the list traffic into a folder as it arrived so it wasn't in my inbox. (If you're using GMail, it also knows list traffic from non-list traffic by the headers and gives a "reply to list" option usually, as long as the list is writing proper headers into the mail.)
If you really want to be old school, mailman also responds to most of the old list commands that predecessor list processors used. But you have to send them as an email to the list processing address, and not the list reply address with Mailman. Both addresses are also usually listed at the bottom of the email. Or they're in the headers.
Old school baby.
Mailman also by default has a threaded web reader of all the list archives on its own webpage. That's probably the closest thing you'll get to say, a forum like this, out of a mailing list.
