Postfix, Courier, and backups - appropriate file permissions?

Picture a mail store for virtual users. Arriving mail is managed by Postfix (creating new files); access to mail is provided by Courier IMAP (moving files around, deleting files, creating sent mail); and periodically, off-site backups are taken (reading files without changing anything). Obviously this will all work if every process runs as root; equally obviously, they shouldn't all run as root. The current setup has all the files owned by the postfix user and group. The Postfix and Courier processes all run as that uid/gid. But I would like to have the backup process run as a dedicated read-only user. Is there a way to tell Postfix and Courier to create their files as user backup:postfix with permissions 0464/0575? I'm on Debian GNU/Linux if that makes any difference.