What is a reverse DNS entry? Why do I need one?

You already have an 'A' entry in DNS that maps your mail server's name - such as mail.yourcompany.com - to an IP address such as 1.2.3.4. This looks like the following:

mail.yourcompany.com. A 1.2.3.4

A 'reverse' entry, some times referred to as a 'PTR' record, does the opposite, meaning that it maps the IP address of your mail server (1.2.3.4) to a host name such as mail.yourcompany.com. That DNS entry looks like this:

1.2.3.4  PTR mail.yourcompany.com.

This reverse entry must be made by whoever controls the IP address - almost always your ISP - rather than by whoever controls the domain.

When you send outbound mail, some receiving mail servers will detect the IP address that your mail is being sent from, and look up the reverse DNS record for that IP address to see if it corresponds to the From address of the email you are sending. Those receiving mail servers may refuse to accept your message if they see that the email is from [e-mail] but that the IP address of your mail server resolves to, say, something.yourisp.net.

Therefore, we do recommend that your ISP establish a 'reverse DNS' entry for your mail server's IP address. This is not necessary for customers using our service for outbound filtering.