Packages with the mbox use flag

Global definition: Add support for mbox (/var/spool/mail) style mail spools.

mail-client / mutt : A small but very powerful text-based mail client

mail-filter / procmail : Mail delivery agent/filter

mail-mta / postfix : A fast and secure drop-in replacement for sendmail

mail-mta / sendmail : Widely-used Mail Transport Agent (MTA)

net-mail / popa3d : A security oriented POP3 server

net-mail / tpop3d : An extensible POP3 server with vmail-sql/MySQL support