Create separate function to just validate MX records

Some Cloud providers block port 25 in order to prevent email spam, so it might be useful to be able to validate MX records without validating connectivity to MX host.

Author: skolodyazhnyy

checkmail.go

@@ -45,6 +45,16 @@ func ValidateFormat(email string) error {
 	return nil
 }
 
+// ValidateMX validate if MX record exists for a domain.
+func ValidateMX(email string) error {
+	_, host := split(email)
+	if _, err := net.LookupMX(host); err != nil {
+		return ErrUnresolvableHost
+	}
+
+	return nil
+}
+
 // ValidateHost validate mail host.
 func ValidateHost(email string) error {
 	_, host := split(email)