Postfix is your Mail Transfer Agent, which means that Postfix role is to actually send emails from one server to the other. Postfix is the server that will let you send an email, and also the server which will receive emails for you from other email servers.

Dovecot instead, is the IMAP server, which will let you access your email, store it.

You need to setup some specific USE flags before installing Postfix and Dovecot

USE flags:

echo "*/* maildir dovecot sasl" >> /etc/portage/package.use/mailserver
echo "net-mail/dovecot managesieve sqlite lz4" >> /etc/portage/package.use/mailserver
echo "mail-mta/postfix dovecot-sasl sqlite -sasl" >> /etc/portage/package.use/mailserver 

This is needed to ensure that the storage format is mailbox, that we will need SQLite support and we want SASL authentication.

Emerge the servers:

emerge -vp postfix dovecot