Add option to validate MX record (#125)

README.md

@@ -266,6 +266,18 @@ You can require that your `EmailValidator` reject all emails that have obsolete
 JMail.validator().disallowObsoleteWhitespace();
 ```
 
+#### Require a valid MX record
+
+You can require that your `EmailValidator` reject all email addresses that do not have a valid MX
+record associated with the domain.
+
+> **Please note that since this rule looks up DNS records, including this rule on your email validator can significantly increase the
+amount of time it takes to validate email addresses.**
+
+```java
+JMail.validator().requireValidMXRecord();
+```
+
 ### Bonus: IP Address Validation
 
 Since validating email addresses requires validation of IP addresses,

checkstyle.xml

@@ -194,7 +194,7 @@
     <module name="OverloadMethodsDeclarationOrder"/>
     <module name="VariableDeclarationUsageDistance"/>
     <module name="ImportOrder">
-      <property name="groups" value="*,com,jakarta,java,net,nl,org"/>
+      <property name="groups" value="*,com,jakarta,java,javax,net,nl,org"/>
       <property name="ordered" value="true"/>
       <property name="separated" value="true"/>
       <property name="option" value="bottom"/>

pom.xml

@@ -323,11 +323,5 @@
       <version>${junit.version}</version>
       <scope>test</scope>
     </dependency>
-    <dependency>
-      <groupId>org.junit.platform</groupId>
-      <artifactId>junit-platform-runner</artifactId>
-      <version>${junit.platform.version}</version>
-      <scope>test</scope>
-    </dependency>
   </dependencies>
 </project>

src/main/java/com/sanctionco/jmail/EmailValidator.java

@@ -42,6 +42,8 @@ public final class EmailValidator {
       = ValidationRules::disallowReservedDomains;
   private static final Predicate<Email> DISALLOW_OBSOLETE_WHITESPACE_PREDICATE
       = ValidationRules::disallowObsoleteWhitespace;
+  private static final Predicate<Email> REQUIRE_VALID_MX_RECORD_PREDICATE
+      = ValidationRules::requireValidMXRecord;
 
   private final Set<Predicate<Email>> validationPredicates;
 
@@ -189,6 +191,20 @@ public EmailValidator disallowObsoleteWhitespace() {
     return withRule(DISALLOW_OBSOLETE_WHITESPACE_PREDICATE);
   }
 
+  /**
+   * Create a new {@code EmailValidator} with all rules from the current instance and the
+   * {@link ValidationRules#requireValidMXRecord(Email)} rule.
+   * Email addresses that have a domain without a valid MX record will fail validation.
+   *
+   * <p><strong>NOTE: Adding this rule to your EmailValidator may significantly increase
+   * the amount of time it takes to validate email addresses.</strong>
+   *
+   * @return the new {@code EmailValidator} instance
+   */
+  public EmailValidator requireValidMXRecord() {
+    return withRule(REQUIRE_VALID_MX_RECORD_PREDICATE);
+  }
+
   /**
    * Return true if the given email address is valid according to all registered validation rules,
    * or false otherwise. See {@link JMail#tryParse(String)} for details on the basic

src/main/java/com/sanctionco/jmail/ValidationRules.java

@@ -1,5 +1,7 @@
 package com.sanctionco.jmail;
 
+import com.sanctionco.jmail.dns.DNSLookupUtil;
+
 import java.util.Arrays;
 import java.util.HashSet;
 import java.util.List;
@@ -136,4 +138,14 @@ public static boolean disallowReservedDomains(Email email) {
 
     return true;
   }
+
+  /**
+   * Rejects an email address that does not have a valid MX record for the domain.
+   *
+   * @param email the email address to validate
+   * @return true if this email address has a valid MX record, or false if it does not
+   */
+  public static boolean requireValidMXRecord(Email email) {
+    return DNSLookupUtil.hasMXRecord(email.domainWithoutComments());
+  }
 }

src/main/java/com/sanctionco/jmail/dns/DNSLookupUtil.java

@@ -0,0 +1,40 @@
+package com.sanctionco.jmail.dns;
+
+import java.util.Hashtable;
+
+import javax.naming.NamingException;
+import javax.naming.directory.Attribute;
+import javax.naming.directory.DirContext;
+import javax.naming.directory.InitialDirContext;
+
+/**
+ * Utility class that provides static methods for DNS related operations.
+ */
+public final class DNSLookupUtil {
+
+  /**
+   * Private constructor to prevent instantiation.
+   */
+  private DNSLookupUtil() {
+  }
+
+  /**
+   * Determine if the given domain has a valid MX record.
+   *
+   * @param domain the domain whose MX record to check
+   * @return true if the domain has a valid MX record, or false if it does not
+   */
+  public static boolean hasMXRecord(String domain) {
+    Hashtable<String, String> env = new Hashtable<>();
+    env.put("java.naming.factory.initial", "com.sun.jndi.dns.DnsContextFactory");
+
+    try {
+      DirContext ctx = new InitialDirContext(env);
+      Attribute attr = ctx.getAttributes(domain, new String[]{"MX"}).get("MX");
+
+      return attr != null && attr.size() > 0;
+    } catch (NamingException e) {
+      return false;
+    }
+  }
+}

src/main/java/module-info.java

@@ -4,6 +4,9 @@
  * @author Rohan Nagar (rohannagar11@gmail.com)
  */
 open module com.sanctionco.jmail {
+  requires java.naming;
+
   exports com.sanctionco.jmail;
+  exports com.sanctionco.jmail.dns;
   exports com.sanctionco.jmail.net;
 }

src/test/java/com/sanctionco/jmail/EmailValidatorTest.java

@@ -182,6 +182,24 @@ void allowsOtherAddresses(String email) {
     }
   }
 
+  @Nested
+  class RequireValidMXRecord {
+    @ParameterizedTest(name = "{0}")
+    @ValueSource(strings = {
+        "test@domain.test", "test@domain.example", "test@domain.invalid", "test@domain.localhost"})
+    void rejectsDomainsWithoutMXRecord(String email) {
+      runInvalidTest(JMail.validator().requireValidMXRecord(), email);
+    }
+
+    @ParameterizedTest(name = "{0}")
+    @ValueSource(strings = {
+        "test@gmail.com", "test@hotmail.com", "test@yahoo.com", "test@utexas.edu",
+        "test@gmail.(comment)com"})
+    void allowsDomansWithMXRecord(String email) {
+      runValidTest(JMail.validator().requireValidMXRecord(), email);
+    }
+  }
+
   @Nested
   class CustomRule {
     @ParameterizedTest(name = "{0}")

src/test/java/com/sanctionco/jmail/dns/DNSLookupUtilTest.java

@@ -0,0 +1,22 @@
+package com.sanctionco.jmail.dns;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+class DNSLookupUtilTest {
+
+  @Test
+  void canLookupValidMXRecord() {
+    assertThat(DNSLookupUtil.hasMXRecord("gmail.com")).isTrue();
+    assertThat(DNSLookupUtil.hasMXRecord("yahoo.com")).isTrue();
+    assertThat(DNSLookupUtil.hasMXRecord("hotmail.com")).isTrue();
+    assertThat(DNSLookupUtil.hasMXRecord("utexas.edu")).isTrue();
+  }
+
+  @Test
+  void failsToFindInvalidMXRecord() {
+    assertThat(DNSLookupUtil.hasMXRecord("a.com")).isFalse();
+    assertThat(DNSLookupUtil.hasMXRecord("whatis.hello")).isFalse();
+  }
+}