rDNS for mail-only server
I have a Shopify store, and I've been having some problems with my replies to customer support requests going into their spam folders. One of the things I'm trying to do is set up my linode's reverse DNS.
The problem is, the site (retrostuff.ca) isn't hosted on my linode, it's hosted on Shopify. Only the mail server is hosted on my linode. If I try to point my linode's rDNS to retrostuff.ca, the manager errors out telling me the forward/reverse must match. If I try to point my linode's rDNS to mx.retrostuff.ca, then spam test services complain that the mail comes from "retrostuff.ca" but the rDNS is mismatched and points to "mx.retrostuff.ca".
What's the normal way to solve this problem?
That certainly is a bit of pickle. One thing to keep in mind is that you can only set a Linode's rDNS to match an A record that's configured in the Linode DNS Manager. You can read a bit about that here.
While it involves a few steps, I think what you would need to do is migrate your DNS records to Linode. Then you'll point the blank A record to the current, Shopify, IP for "retrostuff.ca." And next create a subdomain for "mx.retrostuff.ca" which points to your Linode's IP. There is some more information on formatting A records here.
This will allow you to set the rDNS and keep the services separated.
If you don't want to move your DNS zone you could consider using a third-party email service, like Google which doesn't require a rDNS.