support domain key records

tests/validations.test.js

@@ -68,6 +68,7 @@ describe('validateDomainData', () => {
     { ...defaultDomain, name: '_acme-challenge' },
     { ...defaultDomain, name: '_dmarc' },
     { ...defaultDomain, name: '_gh-is-a-dev' },
+    { ...defaultDomain, name: '_domainkey' },
   ];
 
   const validCases = [
@@ -104,6 +105,8 @@ describe('validateDomainData', () => {
     { ...defaultDomain, name: '_gh-phenax.akshay' },
     { ...defaultDomain, name: '_gh-hello01-ga' },
     { ...defaultDomain, name: '_gh-hello01_ga' },
+    { ...defaultDomain, name: '_domainkey.subdomain' },
+    { ...defaultDomain, name: 'mx._domainkey.subdomain' },
   ];
 
   it('should return false for invalid data', () => {