When running "useradd -m user" in a clean stage3 tarball, you get the following message: Creating mailbox file: No such file or directory This is likely because MAIL_DIR is set to /var/spool/mail in /etc/login.defs, but the directory doesn't exist.
I figure the proper solution would be making the directory in sys-apps/baselayout[build] or similar, or having shadow depend on a package that installs that directory, such as acct-user/mail