[This document is a copy taken from http://marc.merlins.org/netrants/nullenvelope.txt on 2001-03-28. It was written by Marc Merlin.] Your were probably pointed to this URL because your mail server is configured in violation of rfc1123. Your server probably does something like this: > 220 X1 NT-ESMTP Server mailhost.directlink.net (IMail 6.05 1039005-4) > mail from: <> > 501 bogus mail from > quit > 221 Goodbye or this > 220-rocmail.com ESMTP MDaemon 3.5.6 ready > 220-RocSoft does not allow you to relay mail. No FakeMail > 220-No SPAM, No USBE, No Telnet, Tchuessie! > 220 > helo foo > 250 rocmail.com Hello foo, pleased to meet you > mail from: <> > 550 Sorry, this server is configured to refuse this sort of mail (to combat the SPAM problem) > quit > 221 See ya in cyberspace ftp://ftp.isi.edu/in-notes/rfc1123.txt section 5.2.9 states: ---- The syntax shown in RFC-821 for the MAIL FROM: command omits the case of an empty path: "MAIL FROM: <>" (see RFC-821 Page 15). An empty reverse path MUST be supported. ---- Refusing mail from an NULL envelope sender: - breaks the above RFC - accomplishes very little with regards to spam prevention - prevents you from getting any bounce messages back if one of your users Emails other people with a bad address. Your server then clogs up the mail spool of the servers that are trying to send you a bounce by refusing those bounces - Last but not least, you are now going to have your Email rejected by servers like sourceforge.net or valinux.com because they require validation of your envelope and header sender. Those mail servers connect back to you and check the address like this: > 220 server ESMTP Sendmail 8.9.3/8.9.3/Marc v3.1 (09/04/98); Tue, 27 Mar 2001 16:08:39 -0800 > helo foo > 250 server Hello root@connectingserver [X.X.X.X], pleased to meet you > mail from: <> > 250 <>... Sender ok > rcpt to: > 250 ... Recipient ok > quit > 221 server closing connection Because you refuse the "mail from: <>", our mail server refuses your mail.