Several users, same mailbox

[Adrian]

Hello,

I have had Dovecot set up to use PAM authentication, as my current users
all have system accounts on Debian.

I've just moved over to passwdfile authentication, because I want to
add new users who won't have logins on the system.  This is working
fine with my existing users.

Now, the problem is that I want the new users to share the SAME
(Maildir) 'mailbox'.

Currently 10-mail.conf contains
mail_location = maildir:/var/mail/%u:LAYOUT=fs

so this needs to be overridden, or I need some new way of mapping users
to mail locations.

I could probably do it with symlinks from /var/mail/newuser
to /var/mail/mail-location

but there may be a better way to do it than that.  I'd like it to be in
a file, not needing mySQL or other database.

Lastly, are there any implications for locking?  These will be used
rarely but if a second user logs in for the same mailbox they should
get a lock error.

dovecot -n
# 2.3.4.1 (f79e8e7e4): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.5.4 ()
# OS: Linux 4.19.0-11-amd64 x86_64 Debian 10.7 ext4
# Hostname: example.com
disable_plaintext_auth = no
mail_location = maildir:/var/mail/%u:LAYOUT=fs

Thanks in advance