diff --git tests/phpunit/includes/mock-mailer.php tests/phpunit/includes/mock-mailer.php
index c91a67a..7acfd57 100644
--- tests/phpunit/includes/mock-mailer.php
+++ tests/phpunit/includes/mock-mailer.php
@@ -17,7 +17,7 @@ class MockPHPMailer extends PHPMailer {
 			'to'      => $this->to,
 			'cc'      => $this->cc,
 			'bcc'     => $this->bcc,
-			'header'  => $this->MIMEHeader,
+			'header'  => $this->MIMEHeader . $this->mailHeader,
 			'subject' => $this->Subject,
 			'body'    => $this->MIMEBody,
 		);
diff --git tests/phpunit/tests/mail.php tests/phpunit/tests/mail.php
index 5142c72..27bc739 100644
--- tests/phpunit/tests/mail.php
+++ tests/phpunit/tests/mail.php
@@ -307,4 +307,40 @@ class Tests_Mail extends WP_UnitTestCase {
 
 		$this->assertNotContains( 'quoted-printable', $GLOBALS['phpmailer']->mock_sent[0]['header'] );
 	}
+
+	/**
+	 * @ticket 21659
+	 */
+	public function test_wp_mail_addresses_arent_encoded() {
+		$to      = 'Lukáš To <to@example.org>';
+		$subject = 'Testing #21659';
+		$message = 'Only the name should be encoded, not the address.';
+
+		$headers = array(
+			'From'     => 'From: Lukáš From <from@example.org>',
+			'Cc'       => 'Cc: Lukáš CC <cc@example.org>',
+			'Bcc'      => 'Bcc: Lukáš BCC <bcc@example.org>',
+			'Reply-To' => 'Reply-To: Lukáš Reply-To <reply_to@example.org>',
+		);
+
+		$expected = array(
+			'To'       => 'To: =?UTF-8?B?THVrw6HFoSBUbw==?= <to@example.org>',
+			'From'     => 'From: =?UTF-8?Q?Luk=C3=A1=C5=A1_From?= <from@example.org>',
+			'Cc'       => 'Cc: =?UTF-8?B?THVrw6HFoSBDQw==?= <cc@example.org>',
+			'Bcc'      => 'Bcc: =?UTF-8?B?THVrw6HFoSBCQ0M=?= <bcc@example.org>',
+			'Reply-To' => 'Reply-To: =?UTF-8?Q?Luk=C3=A1=C5=A1_Reply-To?= <reply_to@example.org>',
+		);
+
+		wp_mail( $to, $subject, $message, array_values( $headers ) );
+
+		/** @var MockPHPMailer $mailer */
+		$mailer        = tests_retrieve_phpmailer_instance();
+		$sent_headers  = preg_split( "/\r\n|\n|\r/", $mailer->get_sent()->header );
+		$headers['To'] = "To: $to";
+
+		foreach ( $headers as $header => $value ) {
+			$target_headers = preg_grep( "/^$header:/", $sent_headers );
+			$this->assertEquals( $expected[ $header ], array_pop( $target_headers ) );
+		}
+	}
 }