I feel a bit dumb here.
On my Redhat systems, when I open a root shell and have the message "You have new mail in /root", I just type "mail" and there I am reading the messages on the command line.
With Gentoo, I get the "You have new mail..." message, but I don't have the proper application to read it (and do not know what to emerge).
When that happens with me, I usually type mutt and read them, but mutt is often installed by default with many systems. I gather there is no default with Gentoo, but I'm willing to bet that if you emerge mutt you'll be able to read them.
email@example.com mailing list