diff --git a/src/EmailChecker/EmailChecker.php b/src/EmailChecker/EmailChecker.php
index 14e2ba3..4a81250 100644
--- a/src/EmailChecker/EmailChecker.php
+++ b/src/EmailChecker/EmailChecker.php
@@ -54,6 +54,24 @@ public function isValid($email)
             return false;
         }
 
-        return !$this->adapter->isThrowawayDomain($domain);
+        foreach ($this->allDomainSuffixes($domain) as $domainSuffix) {
+            if ($this->adapter->isThrowawayDomain($domainSuffix)) {
+                return false;
+            }
+        }
+
+        return true;
+    }
+
+    /**
+     * @return iterable<string>
+     */
+    private function allDomainSuffixes(string $domain): iterable
+    {
+        $components = explode('.', $domain);
+
+        foreach ($components as $i => $_) {
+            yield implode('.', array_slice($components, $i));
+        }
     }
 }
diff --git a/tests/EmailChecker/Tests/EmailCheckerTest.php b/tests/EmailChecker/Tests/EmailCheckerTest.php
index 78ce8e6..849b644 100644
--- a/tests/EmailChecker/Tests/EmailCheckerTest.php
+++ b/tests/EmailChecker/Tests/EmailCheckerTest.php
@@ -12,17 +12,14 @@
 namespace EmailChecker\Tests;
 
 use EmailChecker\Adapter\AdapterInterface;
+use EmailChecker\Adapter\ArrayAdapter;
 use EmailChecker\EmailChecker;
 
 final class EmailCheckerTest extends TestCase
 {
     public function testEmailIsValid(): void
     {
-        $adapter = $this->createMock(AdapterInterface::class);
-        $adapter
-             ->method('isThrowawayDomain')
-             ->willReturn(false);
-
+        $adapter = new ArrayAdapter([]);
         $checker = new EmailChecker($adapter);
 
         self::assertTrue($checker->isValid('foo@bar.org'));
@@ -30,11 +27,7 @@ public function testEmailIsValid(): void
 
     public function testEmailIsNotValid(): void
     {
-        $adapter = $this->createMock(AdapterInterface::class);
-        $adapter
-             ->method('isThrowawayDomain')
-             ->willReturn(true);
-
+        $adapter = new ArrayAdapter(['bar.org']);
         $checker = new EmailChecker($adapter);
 
         self::assertFalse($checker->isValid('foo@bar.org'));
@@ -42,9 +35,17 @@ public function testEmailIsNotValid(): void
 
     public function testMalformattedEmail(): void
     {
-        $adapter = $this->createMock(AdapterInterface::class);
+        $adapter = new ArrayAdapter([]);
         $checker = new EmailChecker($adapter);
 
         self::assertFalse($checker->isValid('foo[at]bar.org'));
     }
+
+    public function testSubDomain(): void
+    {
+        $adapter = new ArrayAdapter(['bar.org']);
+        $checker = new EmailChecker($adapter);
+
+        self::assertFalse($checker->isValid('foo@baz.bar.org'));
+    }
 }