Fold long address headers

addrAppend() joined all addresses onto one line, so a To, Cc, Bcc or
Reply-To header with many recipients could go past the 998 character
limit in RFC 5322. Fold between addresses when the next one would not
fit. Short lists are unchanged.

Fixes #2690

src/PHPMailer.php

@@ -2647,7 +2647,21 @@ class PHPMailer
             $addresses[] = $this->addrFormat($address);
         }
 
-        return $type . ': ' . implode(', ', $addresses) . static::$LE;
+        //Fold between addresses so that no line exceeds the RFC 5322 limit,
+        //leaving room for the comma at the end of a folded line
+        $header = $type . ': ' . array_shift($addresses);
+        $lineLength = strlen($header);
+        foreach ($addresses as $address) {
+            if ($lineLength + strlen($address) + 3 > static::MAX_LINE_LENGTH) {
+                $header .= ',' . static::$LE . ' ' . $address;
+                $lineLength = strlen($address) + 1;
+            } else {
+                $header .= ', ' . $address;
+                $lineLength += strlen($address) + 2;
+            }
+        }
+
+        return $header . static::$LE;
     }
 
     /**

test/PHPMailer/AddrAppendTest.php

@@ -0,0 +1,64 @@
+<?php
+
+/**
+ * PHPMailer - PHP email transport unit tests.
+ * PHP version 5.5.
+ *
+ * @author    Marcus Bointon <phpmailer@synchromedia.co.uk>
+ * @author    Andy Prevost
+ * @copyright 2012 - 2020 Marcus Bointon
+ * @copyright 2004 - 2009 Andy Prevost
+ * @license   https://www.gnu.org/licenses/old-licenses/lgpl-2.1.html GNU Lesser General Public License
+ */
+
+namespace PHPMailer\Test\PHPMailer;
+
+use PHPMailer\PHPMailer\PHPMailer;
+use PHPMailer\Test\TestCase;
+
+/**
+ * Test creating address headers.
+ *
+ * @covers \PHPMailer\PHPMailer\PHPMailer::addrAppend
+ */
+final class AddrAppendTest extends TestCase
+{
+    /**
+     * A short address list stays on one line.
+     */
+    public function testShortListIsNotFolded()
+    {
+        $header = $this->Mail->addrAppend('To', [['joe@example.com', 'Joe'], ['zoe@example.com', '']]);
+
+        self::assertSame('To: Joe <joe@example.com>, zoe@example.com' . PHPMailer::CRLF, $header);
+    }
+
+    /**
+     * A long address list is folded between addresses so no line exceeds the RFC 5322 limit.
+     */
+    public function testLongListIsFolded()
+    {
+        $addresses = [];
+        $formatted = [];
+        for ($i = 1; $i <= 100; ++$i) {
+            $addresses[] = ['recipient' . $i . '@example.com', 'Recipient ' . $i];
+            $formatted[] = 'Recipient ' . $i . ' <recipient' . $i . '@example.com>';
+        }
+
+        $header = $this->Mail->addrAppend('Cc', $addresses);
+        $lines = explode(PHPMailer::CRLF, substr($header, 0, -strlen(PHPMailer::CRLF)));
+
+        self::assertGreaterThan(1, count($lines), 'The header was not folded');
+        foreach ($lines as $index => $line) {
+            self::assertLessThanOrEqual(PHPMailer::MAX_LINE_LENGTH, strlen($line), "Line $index is too long");
+            if ($index > 0) {
+                self::assertSame(' ', $line[0], "Line $index is not a folded continuation");
+            }
+        }
+        self::assertSame(
+            'Cc: ' . implode(', ', $formatted) . PHPMailer::CRLF,
+            str_replace(PHPMailer::CRLF . ' ', ' ', $header),
+            'Unfolding the header did not give back the address list'
+        );
+    }
+}