http://netbsd.i2p/docs/guide/en/chap-mail.html
But for many users
the number of daily messages will increase with time and a more
rational organization of the mail storage will become necessary,
for example subdividing mail in different mail boxes organized
by topic. If, for example, you subscribe to a mailing list, you
will likely receive many messages every day and you will want to
keep them separate from the rest of your mail.