diff --git a/class.phpmailer.php b/class.phpmailer.php
index 8d6c291fe..c891c3d03 100644
--- a/class.phpmailer.php
+++ b/class.phpmailer.php
@@ -570,6 +570,12 @@ class PHPMailer
      */
     const CRLF = "\r\n";
 
+    /**
+     * The maximum line length allowed by RFC 2822 section 2.1.1
+     * @type integer
+     */
+    const MAX_LINE_LENGTH = 998;
+
     /**
      * Constructor.
      * @param boolean $exceptions Should we throw external exceptions?
@@ -1001,8 +1007,9 @@ public function preSend()
                 throw new phpmailerException($this->lang('empty_message'), self::STOP_CRITICAL);
             }
 
-            $this->MIMEHeader = $this->createHeader();
+            //Create body before headers in case body makes changes to headers (e.g. altering transfer encoding)
             $this->MIMEBody = $this->createBody();
+            $this->MIMEHeader = $this->createHeader();
 
             // To capture the complete message when using mail(), create
             // an extra header list which createHeader() doesn't fold in
@@ -1823,16 +1830,30 @@ public function createBody()
 
         $bodyEncoding = $this->Encoding;
         $bodyCharSet = $this->CharSet;
+        //Can we do a 7-bit downgrade?
         if ($bodyEncoding == '8bit' and !$this->has8bitChars($this->Body)) {
             $bodyEncoding = '7bit';
             $bodyCharSet = 'us-ascii';
         }
+        //If lines are too long, change to quoted-printable transfer encoding
+        if (self::hasLineLongerThanMax($this->Body)) {
+            $this->Encoding = 'quoted-printable';
+            $bodyEncoding = 'quoted-printable';
+            $bodyCharSet = 'us-ascii'; //qp always fits into ascii
+        }
+
         $altBodyEncoding = $this->Encoding;
         $altBodyCharSet = $this->CharSet;
+        //Can we do a 7-bit downgrade?
         if ($altBodyEncoding == '8bit' and !$this->has8bitChars($this->AltBody)) {
             $altBodyEncoding = '7bit';
             $altBodyCharSet = 'us-ascii';
         }
+        //If lines are too long, change to quoted-printable transfer encoding
+        if (self::hasLineLongerThanMax($this->AltBody)) {
+            $altBodyEncoding = 'quoted-printable';
+            $altBodyCharSet = 'us-ascii';
+        }
         switch ($this->message_type) {
             case 'inline':
                 $body .= $this->getBoundary($this->boundary[1], $bodyCharSet, '', $bodyEncoding);
@@ -3371,6 +3392,18 @@ public function DKIM_Add($headers_line, $subject, $body)
         return $dkimhdrs . $signed . "\r\n";
     }
 
+    /**
+     * Detect if a string contains a line longer than the maximum line length allowed.
+     * @param $str
+     * @return boolean
+     * @static
+     */
+    public static function hasLineLongerThanMax($str)
+    {
+        //+2 to include CRLF line break for a 1000 total
+        return (boolean)preg_match('/^(.{'.(self::MAX_LINE_LENGTH + 2).',})/m', $str);
+    }
+
     /**
      * Allows for public read access to 'to' property.
      * @access public
diff --git a/test/phpmailerTest.php b/test/phpmailerTest.php
index 4d519cb4f..c3247dad7 100644
--- a/test/phpmailerTest.php
+++ b/test/phpmailerTest.php
@@ -146,12 +146,12 @@ public function buildBody()
 
         // Determine line endings for message
         if ($this->Mail->ContentType == 'text/html' || strlen($this->Mail->AltBody) > 0) {
-            $eol = '<br/>';
+            $eol = "<br/>". PHPMailer::CRLF;
             $bullet = '<li>';
             $bullet_start = '<ul>';
             $bullet_end = '</ul>';
         } else {
-            $eol = "\n";
+            $eol = PHPMailer::CRLF;
             $bullet = ' - ';
             $bullet_start = '';
             $bullet_end = '';
@@ -1040,6 +1040,57 @@ public function testEmptyBody()
         $this->assertFalse($this->Mail->send(), $this->Mail->ErrorInfo);
     }
 
+    /**
+     * Test constructing a message that contains lines that are too long for RFC compliance.
+     */
+    public function testLongBody()
+    {
+        $oklen = str_repeat(str_repeat('0', PHPMailer::MAX_LINE_LENGTH) . PHPMailer::CRLF, 10);
+        $badlen = str_repeat(str_repeat('1', PHPMailer::MAX_LINE_LENGTH + 1) . PHPMailer::CRLF, 2);
+
+        $this->Mail->Body = "This message contains lines that are too long.".
+            PHPMailer::CRLF . PHPMailer::CRLF . $oklen . $badlen . $oklen;
+        $this->assertTrue(
+            PHPMailer::hasLineLongerThanMax($this->Mail->Body),
+            'Test content does not contain long lines!'
+        );
+        $this->buildBody();
+        $this->Mail->Encoding = '8bit';
+        $this->Mail->preSend();
+        $message = $this->Mail->getSentMIMEMessage();
+        $this->assertFalse(PHPMailer::hasLineLongerThanMax($message), 'Long line not corrected.');
+        $this->assertContains(
+            'Content-Transfer-Encoding: quoted-printable',
+            $message,
+            'Long line did not cause transfer encoding switch.'
+        );
+    }
+
+    /**
+     * Test constructing a message that does NOT contain lines that are too long for RFC compliance.
+     */
+    public function testShortBody()
+    {
+        $oklen = str_repeat(str_repeat('0', PHPMailer::MAX_LINE_LENGTH) . PHPMailer::CRLF, 10);
+
+        $this->Mail->Body = "This message does not contain lines that are too long.".
+            PHPMailer::CRLF . PHPMailer::CRLF . $oklen;
+        $this->assertFalse(
+            PHPMailer::hasLineLongerThanMax($this->Mail->Body),
+            'Test content contains long lines!'
+        );
+        $this->buildBody();
+        $this->Mail->Encoding = '8bit';
+        $this->Mail->preSend();
+        $message = $this->Mail->getSentMIMEMessage();
+        $this->assertFalse(PHPMailer::hasLineLongerThanMax($message), 'Long line not corrected.');
+        $this->assertNotContains(
+            'Content-Transfer-Encoding: quoted-printable',
+            $message,
+            'Short line caused transfer encoding switch.'
+        );
+    }
+
     /**
      * Test keepalive (sending multiple messages in a single connection)
      */
@@ -1288,6 +1339,20 @@ public function testLineBreaks()
         $this->assertEquals($target, PHPMailer::normalizeBreaks($mixedsrc), 'Mixed break reformatting failed');
     }
 
+    /**
+     * Test line length detection
+     */
+    public function testLineLength()
+    {
+        $oklen = str_repeat(str_repeat('0', PHPMailer::MAX_LINE_LENGTH)."\r\n", 10);
+        $badlen = str_repeat(str_repeat('1', PHPMailer::MAX_LINE_LENGTH + 1) . "\r\n", 2);
+        $this->assertTrue(PHPMailer::hasLineLongerThanMax($badlen), 'Long line not detected (only)');
+        $this->assertTrue(PHPMailer::hasLineLongerThanMax($oklen . $badlen), 'Long line not detected (first)');
+        $this->assertTrue(PHPMailer::hasLineLongerThanMax($badlen . $oklen), 'Long line not detected (last)');
+        $this->assertTrue(PHPMailer::hasLineLongerThanMax($oklen . $badlen . $oklen), 'Long line not detected (middle)');
+        $this->assertFalse(PHPMailer::hasLineLongerThanMax($oklen), 'Long line false positive');
+    }
+
     /**
      * Test setting and retrieving message ID
      */