On Thu, 18 Sep 2003 00:36:05 EDT, David B Harris <david@eelf.ddts.net> said:
If they don't accept anything on port 25, either by sending all packets to /dev/null or by responding with SYN+RST ("Connection refused"), MTAs everywhere will consider this a "temporary error."
They could save us a bunch of RTTs, remove concerns they were harvesting the MAIL FROM and RCPT TO, and lower their load if they just implemented RFC1846 and replied '521 64.94.110.11 Fleep Off'
From RFC1846:
1. Motivations Hosts on the Internet have shifted from large, general-purpose hosts to smaller, more specialized hosts. There is an increasing number of hosts which are dedicated to specific tasks, such as serving NTP or DNS. These dedicated hosts frequently do not provide mail service. Usually, these mailless hosts do not run an SMTP server. Unfortunately, users will occasionally misaddress mail to these hosts. Regular SMTP clients attempting to deliver this misaddressed mail must treat the lack of an SMTP server on the host as a temporary error. They must queue the mail for later delivery, should an SMTP server be started at a later time. This causes the mail to remain queued for days, until it is returned with what is usually a confusing error message.