Issue #3270647 by P44T, AndyF, Sander Edwards van Muijen, Nitin shrivastava, Albert Volkman, mr.baileys, jshimota01, catch, mpellegrin, poker10, alexpott: PhpMail : broken mail headers in PHP 8.0+ because of LF characters

core/lib/Drupal/Core/Mail/Plugin/Mail/PhpMail.php

@@ -109,10 +109,7 @@ class PhpMail implements MailInterface {
     // line-ending format appropriate for your system. If you need to
     // override this, adjust $settings['mail_line_endings'] in settings.php.
     $mail_body = preg_replace('@\r?\n@', $line_endings, $message['body']);
-    // For headers, PHP's API suggests that we use CRLF normally,
-    // but some MTAs incorrectly replace LF with CRLF. See #234403.
-    $mail_headers = str_replace("\r\n", "\n", $headers->toString());
-    $mail_subject = str_replace("\r\n", "\n", $mail_subject);
+    $mail_headers = $headers->toString();
 
     if (!$this->request->server->has('WINDIR') && !str_contains($this->request->server->get('SERVER_SOFTWARE'), 'Win32')) {
       // On most non-Windows systems, the "-f" option to the sendmail command

core/tests/Drupal/Tests/Core/Mail/Plugin/Mail/PhpMailTest.php

@@ -77,9 +77,6 @@ class PhpMailTest extends UnitTestCase {
       ->onlyMethods(['doMail'])
       ->getMock();
 
-    $mailer->expects($this->once())->method('doMail')
-      ->willReturn(TRUE);
-
     $request = $this->getMockBuilder(Request::class)
       ->disableOriginalConstructor()
       ->getMock();
@@ -95,7 +92,6 @@ class PhpMailTest extends UnitTestCase {
     $reflection_property = $reflection->getProperty('request');
     $reflection_property->setAccessible(TRUE);
     $reflection_property->setValue($mailer, $request);
-
     return $mailer;
   }
 
@@ -116,20 +112,43 @@ class PhpMailTest extends UnitTestCase {
       'langcode' => 'en',
       'params' => [],
       'send' => TRUE,
-      'subject' => '',
+      'subject' => "test\r\nsubject",
       'body' => '',
       'headers' => [
         'MIME-Version' => '1.0',
         'Content-Type' => 'text/plain; charset=UTF-8; format=flowed; delsp=yes',
         'Content-Transfer-Encoding' => '8Bit',
         'X-Mailer' => 'Drupal',
-        'Return-Path' => 'from@example.org',
         'From' => '"Foo, Bar, and Baz" <from@example.org>',
         'Reply-to' => 'from@example.org',
+        'Return-Path' => 'from@example.org',
       ],
     ];
 
     $mailer = $this->createPhpMailInstance();
+
+    // Verify we use line endings consistent with the PHP mail() function, which
+    // changed with PHP 8. See:
+    // - https://www.drupal.org/node/3270647
+    // - https://bugs.php.net/bug.php?id=81158
+    $line_end = "\r\n";
+
+    $expected_headers = "MIME-Version: 1.0$line_end";
+    $expected_headers .= "Content-Type: text/plain; charset=UTF-8; format=flowed; delsp=yes$line_end";
+    $expected_headers .= "Content-Transfer-Encoding: 8Bit$line_end";
+    $expected_headers .= "X-Mailer: Drupal$line_end";
+    $expected_headers .= "From: \"Foo, Bar, and Baz\" <from@example.org>$line_end";
+    $expected_headers .= "Reply-to: from@example.org$line_end";
+
+    $mailer->expects($this->once())->method('doMail')
+      ->with(
+        $this->equalTo('to@example.org'),
+        $this->equalTo("=?utf-8?Q?test?={$line_end} =?utf-8?Q?subject?="),
+        $this->equalTo(''),
+        $this->stringStartsWith($expected_headers),
+      )
+      ->willReturn(TRUE);
+
     $this->assertTrue($mailer->mail($message));
   }