An email address formerly very commonly used as the fake sender address of spam. Reportedly still the default sender address in some spamming programs.
Also a cautionary tale on why spam is bad. As you may know, spammers aren't extremely bright, so they just gave a bogus address as the sender's name - but the problem was, they chose a real and live domain.
As a result, public.com was flooded by E-mail - bounces, replies from angry recipients who thought the address was real, and so on. It became one of the first real casualties in the great big spam wars.
public.com now only warns that spams that claim to come from there don't really come there - as that is all they can do.