Some people shouldn't be allowed to run an e-mail system:
From RFC 2821:
Any system that includes an SMTP server supporting mail relaying or delivery MUST support the reserved mailbox "postmaster" as a case- insensitive local name. [...] The requirement to accept mail for postmaster implies that RCPT commands which specify a mailbox for postmaster at any of the domains for which the SMTP server provides mail service, as well as the special case of "RCPT TO:
" (with no domain specification), MUST be supported.
SMTP systems are expected to make every reasonable effort to accept mail directed to Postmaster from any other system on the Internet.
For those who don't know what "postmaster" is for, read the wikipedia article.