According to one bug wrangler on Gentoo Bugzilla, you're a stupid person if you don't read the messages spitted out by ebuilds and run into failures because of that. Of course, no sane person watches the complete compilations of application on the lookout for warnings or other informative messages. But it would be nice if you get a nice summary of all messages in your mailbox after an emerge command.

We are going to set this up, together with Postfix. However, if you already have a Mail Transfer Agent (MTA) running on your system, there's no need to do so of course. You can right away proceed with the last step, step 7.

1. emerge postfix
2. Now, we are going to configure it. Open /etc/postfix/main.cf with your favorite editor (which is nano of course).
3. Pay attention to the following lines:
   
   inet_interfaces = localhost
   home_mailbox = .maildir/

   The other defaults should suffice I'd say.

4. Make sure that your aliases are correct. Open the file /etc/mail/aliases and fill in the entries for root and operator:
   
   root: youruser
   operator: youruser
5. Run the following command as root:
   
   # newaliases

   which will create the file /etc/mail/aliases.db , used by Postfix.

6. Start Postfix:
   
   /etc/init.d/postfix start

   Or add it to your runlevel to make it start automatically upon each boot:

   rc-update add postfix default
7. Postfix is up and running, now we're going to tell Portage that we'd like to send the messages by mail. Simply add the following lines to /etc/make.conf:
   
   PORTAGE_ELOG_CLASSES="info warn error"
   PORTAGE_ELOG_SYSTEM="mail"
   PORTAGE_ELOG_MAILURI="youruser@localhost /usr/sbin/sendmail"

   And of course make sure to replace youruser with your user name.

This is it, if you finished these steps successfully, you'll get an email whenever there are important things spitted out by ebuilds.