diff --git a/src/PHPMailer.php b/src/PHPMailer.php
index 8f4188d..547024e 100644
--- a/src/PHPMailer.php
+++ b/src/PHPMailer.php
@@ -3320,8 +3320,9 @@ class PHPMailer
                     @unlink($signed);
                     //The message returned by openssl contains both headers and body, so need to split them up
                     $parts = explode("\n\n", $body, 2);
-                    $this->MIMEHeader .= $parts[0] . static::$LE . static::$LE;
-                    $body = $parts[1];
+                    //openssl emits bare LF breaks, which SMTP servers may reject
+                    $this->MIMEHeader .= static::normalizeBreaks($parts[0]) . static::$LE . static::$LE;
+                    $body = static::normalizeBreaks($parts[1]);
                 } else {
                     @unlink($signed);
                     throw new Exception(self::lang('signing') . openssl_error_string());
diff --git a/test/PHPMailer/PHPMailerTest.php b/test/PHPMailer/PHPMailerTest.php
index 96d22b0..48a6b42 100644
--- a/test/PHPMailer/PHPMailerTest.php
+++ b/test/PHPMailer/PHPMailerTest.php
@@ -971,6 +971,11 @@ EOT;
 
         $msg = $this->Mail->getSentMIMEMessage();
         self::assertStringNotContainsString("\r\n\r\nMIME-Version:", $msg, 'Incorrect MIME headers');
+        self::assertSame(
+            0,
+            preg_match_all("/(?<!\r)\n/", $msg),
+            'Signed message contains bare line feeds'
+        );
         unlink($certfile);
         unlink($keyfile);
     }