diff --git a/tests/phpunit/includes/testcase.php b/tests/phpunit/includes/testcase.php
index 81f9686050..35d0a62e54 100644
--- a/tests/phpunit/includes/testcase.php
+++ b/tests/phpunit/includes/testcase.php
@@ -53,6 +53,8 @@ class WP_UnitTestCase extends PHPUnit_Framework_TestCase {
 		$this->start_transaction();
 		$this->expectDeprecated();
 		add_filter( 'wp_die_handler', array( $this, 'get_wp_die_handler' ) );
+
+		add_filter( 'wp_mail', array( $this, 'set_wp_mail_globals' ) );
 	}
 
 	/**
@@ -569,4 +571,30 @@ class WP_UnitTestCase extends PHPUnit_Framework_TestCase {
 		$time_array = explode( ' ', $microtime );
 		return array_sum( $time_array );
 	}
+
+	/**
+	 * When `wp_mail()` is called, make sure `$_SERVER['SERVER_NAME']` is faked.
+	 *
+	 * @since 4.3.0
+	 *
+	 * @param array $args `wp_mail()` arguments.
+	 * @return array $args
+	 */
+	public function set_wp_mail_globals( $args ) {
+		if ( ! isset( $_SERVER['SERVER_NAME'] ) ) {
+			$_SERVER['SERVER_NAME'] = 'example.com';
+			add_action( 'phpmailer_init', array( $this, 'tear_down_wp_mail_globals' ) );
+		}
+
+		return $args;
+	}
+
+	/**
+	 * Tear down the faked `$_SERVER['SERVER_NAME']` global used in `wp_mail()`.
+	 *
+	 * @since 4.3.0
+	 */
+	public function tear_down_wp_mail_globals() {
+		unset( $_SERVER['SERVER_NAME'] );
+	}
 }
diff --git a/tests/phpunit/tests/mail.php b/tests/phpunit/tests/mail.php
index 4053966d71..dc000f46e3 100644
--- a/tests/phpunit/tests/mail.php
+++ b/tests/phpunit/tests/mail.php
@@ -6,15 +6,9 @@
 class Tests_Mail extends WP_UnitTestCase {
 	function setUp() {
 		parent::setUp();
-		$_SERVER['SERVER_NAME'] = 'example.com';
 		unset( $GLOBALS['phpmailer']->mock_sent );
 	}
 
-	function tearDown() {
-		unset( $_SERVER['SERVER_NAME'] );
-		parent::tearDown();
-	}
-
 	function test_wp_mail_custom_boundaries() {
 		$to = 'user@example.com';
 		$subject = 'Test email with custom boundaries';