- Mailboxes are generally mode 660
- <tt><var>user</var>:mail</tt> unless the system
- administrator has chosen otherwise. A MUA may remove a
- mailbox (unless it has nonstandard permissions) in which
- case the MTA or another MUA must recreate it if needed.
- Mailboxes must be writable by group mail.
+ Mailboxes are generally either mode 600 and owned by
+ <var>user</var> or mode 660 and owned by
+ <tt><var>user</var>:mail</tt><footnote>
+ There are two traditional permission schemes for mail spools:
+ mode 600 with all mail delivery done by processes running as
+ the destination user, or mode 660 and owned by group mail with
+ mail delivery done by a process running as a system user in
+ group mail. Historically, Debian required mode 660 mail
+ spools to enable the latter model, but that model has become
+ increasingly uncommon and the principle of least privilege
+ indicates that mail systems that use the first model should
+ use permissions of 600. If delivery to programs is permitted,
+ it's easier to keep the mail system secure if the delivery
+ agent runs as the destination user. Debian Policy therefore
+ permits either scheme.
+ </footnote>. The local system administrator may choose a
+ different permission scheme; packages should not make
+ assumptions about the permission and ownership of mailboxes
+ unless required (such as when creating a new mailbox). A MUA
+ may remove a mailbox (unless it has nonstandard permissions) in
+ which case the MTA or another MUA must recreate it if needed.