If you are experiencing difficulties sending email to Horizon Hosting hosted email addresses, the information on this page may assist you in resolving the problem.

As a measure to reduce the quantity of spam (junk mail) received by the email users, Horizon Hosting has added a security measure that checks for what is known as "reverse domain name server lookup" (or ‘reverse DNS lookup"). The feature is applied to all inbound (external) email messages, and is regarded by the internet community as best practice for blocking spam email.

This is because spammers (or junk mailers), who don't want to be known, usually send out spam email via servers that do not ‘resolve' one way or the other.

Unsolicited commercial email messages, or junk email messages ("spam"), is an annoyance to business and home email users. Deleting these messages wastes your valuable time, but the consequences of unsolicited spam email can become far more troublesome as such messages may sometimes contain malicious files or viruses.

Generally speaking, messages are rejected where the inbound message has been sent from a mailserver that does not have "reverse DNS look up" in place. It is considered best practice to block emails from such servers for the reasons stated above.

Thus, under this measure, a mail host with no reverse DNS lookup is regarded as a "spurious" host - and therefore not one that should be trusted as a bona fide origin.

When a message is sent, a mailserver (eg, mail.mailserver.com.au) will ‘reverse resolve' to an IP address, but if the IP address does not resolve back to mail.mailserver.com.au then a ‘no reverse DNS lookup' is the result.

Officially, every mail server on the internet is supposed to resolve in both directions.

The resolve process verifies that the incoming message really is from the source that it reports to be, and if the verification fails, then the inbound mail is automatically rejected.

Horizon Hosting mail servers require that all connecting mail transfer agents have established reverse DNS.

Reverse DNS is a way of associating an IP address with its domain name. The reverse DNS identifier is contained in the PTR portion of the DNS Zone.

If you are on a dynamic IP address, please contact your ISP and request "a static IP address with proper reverse DNS" before attempting to send email to Horizon Hosting through that server.

If you are on a static IP address, please ask your ISP or DNS provider to "establish a reverse DNS record for your IP address, reflecting your correct domain name".

You may also choose to relay your mail through your ISP's mail servers (also called a smart host). They will have reverse DNS setup correctly on their mail servers.