Enable the pop3 plugin, to get notified about new mails.

Packages describing “pop3” as local USE flag

Package “pop3” Flag Description
app-misc/lcd4linux Enable the pop3 plugin, to get notified about new mails.
mail-mta/netqmail Include POP3 server
mail-mta/notqmail Include POP3 server
net-misc/curl Enable Post Office Protocol 3 support

All packages providing a “pop3” USE flag (3)