I have a domain configuration file called example.com.conf
I want to add the following subdomain to it: members.example.conf and add it's own MX records (it's running a seperate mail system).
How do I specify the MX records for a subdomain?
Same way you specify A records. Keep in mind MX records MUST be FQDNs, not IP addresses.
The first column in your file is currently blank because it's inherited from the record above it (likely your SOA record)
This should be just as valid as what I posted above:
Basically, there's more than one way to skin a cat. However, $origins can make things unnecessarily confusing, so sometimes it's better to be verbose.
First, a name like this is not a subdomain, in strict terminology. It is just another name inside the current domain. If it were a subdomain, it would have NS records in the parent domain and and SOA record in the subdomain's file.
If you want to make another name inside your file, all you have to do is simply add it. As others have said, you need to add the MX records for it, using names on the right-hand side. Then add A records for those names. Inside domain "example.com" you could use:
The TTL can be left to default (as I did here).
@ in the first column means "whatever this zone's apex name is" so it will be taken from the configuration file. @ is a lazy way to write the domain's apex, but only that. For example, you cannot use foo.@ to mean foo.example.com -- it won't work.