Every program that accesses mailboxes in the system-wide mail spool directory needs to have write access to it. This is because dot-locking is achieved by creating additional files to that directory, and it must be used (in addition to fcntl() locking) to avoid messing up the mailboxes because of concurrent access.

In Debian, /var/mail is owned by root:mail with mode 2755, and every MUA is setgid mail. See the Debian Policy Manual section 11.6 for more information:

    http://www.debian.org/doc/debian-policy/ch-customized-programs.html#s-mail-transport-agents

If you write a MUA or MTA using Python's mailbox module, your program needs to have write access to /var/mail. That's the only way to do it correctly. It also makes the mailbox module's renaming behavior possible.