Open Relay

Open Relay is a simple mail transfer protocol (SMTP) server that is configured to allow anyone using the Internet to send email messages through it; some servers are configured so that only known users can send email messages. During the 90s, open relay mail servers were common. In fact, once installed, this configuration was default for mail servers on UNIX systems. These server configurations were common because of their simple message transfer method: storing and forwarding email from one computer to another via phone line modems.

Open relay servers have become less common as spammers have become more of a problem in email practices. Spammers leveraged open relay configurations to re-route their own emails through third party email servers to avoid detection. They also used these servers’ information to gain more email addresses for their messages. Once able to access server information, the spammer is able to include a large blind carbon copy list of emails.

While spammers were able to contact a wider range of email users, spam also became easier to detect as each email address received the same spam message. As spam elevated into a larger problem, easily accessed open relay servers became frowned upon. However, because spammers’ practices were well-known, bulk email detection systems were easily created and put in place to stop more simple spammer attempts. As spammers needed more complex messages to reach email users, open relay servers lost their utility to spammers. While malicious senders cannot leverage open relays to send bulk emails, open relays are still vulnerable to address spoofing as these mail servers do not attempt to verify that the owner of an address is an email’s true sender.