What is reverse DNS and why does it matter for email?

Reverse DNS is the lookup that runs in the opposite direction to normal: instead of turning a name into a server address, it turns the address back into a name, using a PTR record. Why it matters for email:

• Receiving mail servers run this check on every connection - a sending server whose address maps back to no name, or to a generic one that doesn’t match who it claims to be, looks exactly like the compromised home machines that send most of the world’s spam, and plenty of providers refuse its mail outright.
• 93% of Australian small business domains pass the reverse DNS check^*, because the businesses sending through Microsoft 365 or Google Workspace inherit their provider’s correctly-configured servers.
• The check matters when you send any other way - a web server emailing order confirmations, or a self-hosted mail server - because the PTR record belongs to whoever controls the IP address, which means the fix is a request to your hosting provider, not an edit in your own DNS.

If mail from your website bounces while mail from your inbox doesn’t, reverse DNS on the web server is the first suspect.