Information Centre

The Information Centre provides some additional useful information on how AOL handles mail which is touched on in our Technical Standards for Email section. Although the information presented here is reasonably technical, our goal is to make this information as clear and accessible as possible.

What is an open relay?

An open relay, also known as third-party relay, is an email server that allows anyone on the Internet to connect to it and send email. For example, 'Joe User', dialled in through 'Big ISP' can connect to the open relay and send his mail to any destination he chooses. This abuse is popular with spammers. For more information, go to open relay.

What is an open proxy?

An open proxy is a server that acts as an open relay but through ports other than 25; open proxies provide surrogate (proxy) communication ability to other servers through ports not reserved for normal use.

What is Reverse DNS and why is it important?

Reverse DNS is a way of associating an IP address with its domain name.

What are IP addresses and how do we use them?

An IP address is a number composed of four parts (called octets) that is used to identify your computer on the internet.

What do AOL error messages mean?

An error message from AOL's email delivery subsystem ( indicates that there was a problem in delivering your message to its destination. When you receive an error from our mail system, your message will be returned to you along with an outline of the mail delivery problem. For information on the content of our mailer-daemon error messages, please go to mailer-daemon error messages.