Why is the default user group "users"? and: rights and access to /var/mail

[Nils Gillmann <ng0@n0.is>]

Hi,

can someone tell me why in gnu/system/shadow module you thought
it would be a good idea to default to "users" as a shared group
for all accounts created as normal user profiles?

Reason why I'm asking has a second question attached:
Why does our opensmtpd-service (and dovecot?) create
/var/mail world readable, owned by root:root?
I'm working on integration of mailx (package done, debugging its
runtime currently[1]), though I think my concern is not exclusive to
mailx: I want users to be able to read mailboxes inside /var/mail
by their name (/var/mail/$username) and which are set to be r+w
only for $username:$username. If you want to list the content of
the folder you would need to be part of the wheel/sudo group,
otherwise you are just able to access your mailbox with your
mailreader.
$username:$username was what I learned as good and secure usage
for user accounts. Why GuixSD uses $username:users is beyond me.
I know recently the default chmod of the user $home was changed
(last year?) so I can no longer read other users homes, but I'm
still questioning the choice.
Some explanation on this would be good.


[1] Not sure if this is something you want to see in Guix master,
as a package and as integration. If you are, I'll work it into
my Guix repo when I'm done with it.