mail_extra_groups=mail setting is often used insecurely to give Dovecot access to create dotlocks to /var/mail directory. If you don't use mboxes in /var/mail, make sure this setting is cleared.

If you do use /var/mail mboxes and Dovecot gives permission errors without it, do one of the following (in the preferred order):

a) Upgrade to v1.0.11 and use the new mail_privileged_group setting instead of mail_extra_groups. b) Make /var/mail sticky and world-writable (chmod 01777 /var/mail) and clear mail_extra_groups setting. c) Make /var/mail sticky (chmod +t /var/mail) and keep mail_extra_groups setting. This fixes the main problem but some may be left.

The mail_privileged_group setting is also available as a patch: http://dovecot.org/patches/1.0/dovecot-1.0.10.mail_priv_groups.diff

mail_extra_groups setting has existed since Dovecot v0.99.10.6. It's never been enabled by default in the distributed dovecot-example.conf, but some distributions enable it by default (at least Debian).

A longer explanation:

The main problem is that if users have filesystem access to the mail server, they can create symlinks. Dovecot doesn't try to prevent following symlinks (and I don't think it should) and normally it isn't a problem. But when mail_extra_groups=mail is set:

1a) Maildir: Any files readable by mail group can be read by the user by symlinking the file to their ~/Maildir/cur/.

1b) Any mbox files readable by mail group can be read by the user by symlinking the mbox file to their ~/mail/ directory.

These are pretty obvious problems and I didn't think they were all that important. Why would anyone have secret files that are mail group-readable? But apparently those do exist in some systems (maybe accidentally).

2a) mbox: Any files/directories under mail group-writable directories can be created/deleted/renamed by symlinking the directory under ~/mail/. For example ln -s /var/mail ~/mail/var, DELETE var/root will happily delete root's mailbox. This I hadn't thought about before.

2b) Maildir: Pretty much the same thing can be done with maildir but with a little less control.

mail_privileged_group setting works by keeping the group in process's saved GID while it's not in use and temporarily switching it to effective GID while dotlocks are created. Currently this is done only when:

1. It's only done for INBOX mbox which doesn't exist under the same location as other mailboxes (so typically under /var/mail).

2. It's used only after initial dotlock creation try failed with EACCES error.