Hi, @ianlunn, I wanted to reply about the following:
Hi @gregraven. My domain is ianlunn.netlify.app. Regardless of the email issue, shouldn’t the reverse DNS lookup return the domain for the IP address? That’s something handled by Netlify rather than my email service, right?
There is no 1:1 connection between IP addresses at Netlify and custom domains. The relationship is one to many. A small set of IP addresses (each IP address being for a different CDN/ADN node) host all the sites at Netlify.
In other words, each individual IP address can and will handle any/all sites we host. So one IP address connects to millions of custom domains. We don’t have a way to returning a specific custom domain for a single IP address. It just isn’t possible at the DNS level.
Regarding how we know which site to show when you browse a Netlify site (the HTTP level), the web browser (or other tool) will send a host HTTP request header which our CDN/ADN node then use to determine which site to display.
For example, if I make a curl request to https://www.netlify.com/ the header curl sends is this:
Host: www.netlify.com
Our support team can help you get the correct DNS records created at Netlify to get your email service working once you tell us what those correct records are. There is more about migrating from another DNS service to Netlify DNS in this support guide.
However, if your emails requires this reverse DNS lookup our Netlify DNS service won’t do this and won’t ever be able to get that working.
If reveres DNS is a hard requirement for your email service (and I don’t know if that is true or not), I would recommend not using Netlify DNS at all. You can still link the custom domain to the site hosted here using the external DNS instructions.
If there are other questions or concerns, please let us know.