What is an open relay?

What is an open relay?

Anyone who has traveled a lot has experienced the following: You check into your hotel. You

connect to the Internet using the Ethernet socket in your hotel room. You try to send an email to

the office, and your email client refuses saying “relaying denied”. What happened? Suppose your

email address is you@foo.bar. Your regular email server, which may be named mail.foo.bar, knows

all of the IP addresses of all of the machines connected to the Internet via the foo.bar domain.

Should the mail.foo.bar forward email coming from another domain than foo.bar, this is referred to

as “relaying”. Most ISPs do not allow relaying of email from untrusted domains, indeed from any

domains but their own. Your laptop computer was using an IP address allocated by your hotel’s DHCP

server. Mail.foo.bar did not recognize this IP address, and refused to relay. There are a lot of

poorly configured email servers however, that will let anyone use them to send email. An open mail

relay becomes a channel for Spam, virtually “hijacked” by unscrupulous spammers who send large

numbers of emails through them until they are discovered and banned, and move on to another open

relay. Early versions of certain email servers did not stop spam email , but defaulted to open

relaying when set up, so that there are many open relays available to spammers today. Recent

versions of most email server products default to denying relaying in order to block junk email.