Dedicated IPs – full-circle reverse DNS and EHLO hostnames on SparkPost

August 14, 2019 Contributors

You may have noticed that recently allocated dedicated IP addresses in your SparkPost account come from specific address ranges.

Unless you are using our Bring your Own IP feature, your dedicated IPs come from a range registered to SparkPost.

SparkPost automatically provides Full-circle reverse DNS, otherwise known as “Forward-confirmed reverse DNS”, for these dedicated IPs.

The IP address resolves back to a specific hostname using a PTR record, which uniquely resolves forward to that same IP address. You can see this using:

The lookup from IP address back to hostname is done via a DNS PTR record. You can see this in full using the dig -x command above without the +short option:

You can also use online tools such as MX Toolbox to view this relationship.

EHLO hostnames in the outbound SMTP conversation

When SparkPost delivers email onwards to a mailbox provider, the SMTP conversation starts with the remote provider announcing themselves, and SparkPost replies with its hostname in the EHLO command, like this. The lines from SparkPost are annotated thus: >

Here’s a network-level message exchange, showing the initial SMTP conversation, captured in WireShark.