diff --git a/examples/extending.phps b/examples/extending.phps
index e7b2d880..c04a731d 100644
--- a/examples/extending.phps
+++ b/examples/extending.phps
@@ -9,6 +9,7 @@
 
 //Import PHPMailer classes into the global namespace
 use PHPMailer\PHPMailer\PHPMailer;
+use PHPMailer\PHPMailer\SMTP;
 use PHPMailer\PHPMailer\Exception;
 
 require '../vendor/autoload.php';
@@ -37,7 +38,7 @@ class myPHPMailer extends PHPMailer
         //Set an HTML and plain-text body, import relative image references
         $this->msgHTML($body, './images/');
         //Show debug output
-        $this->SMTPDebug = 2;
+        $this->SMTPDebug = SMTP::DEBUG_SERVER;
         //Inject a new debug output handler
         $this->Debugoutput = function ($str, $level) {
             echo "Debug level $level; message: $str\n";
diff --git a/examples/gmail.phps b/examples/gmail.phps
index f2c1afc3..1dfb5a93 100644
--- a/examples/gmail.phps
+++ b/examples/gmail.phps
@@ -8,6 +8,7 @@
 
 //Import PHPMailer classes into the global namespace
 use PHPMailer\PHPMailer\PHPMailer;
+use PHPMailer\PHPMailer\SMTP;
 
 require '../vendor/autoload.php';
 
@@ -18,10 +19,10 @@ $mail = new PHPMailer;
 $mail->isSMTP();
 
 //Enable SMTP debugging
-// 0 = off (for production use)
-// 1 = client messages
-// 2 = client and server messages
-$mail->SMTPDebug = 2;
+// SMTP::DEBUG_OFF = off (for production use)
+// SMTP::DEBUG_CLIENT = client messages
+// SMTP::DEBUG_SERVER = client and server messages
+$mail->SMTPDebug = SMTP::DEBUG_SERVER;
 
 //Set the hostname of the mail server
 $mail->Host = 'smtp.gmail.com';
diff --git a/examples/gmail_xoauth.phps b/examples/gmail_xoauth.phps
index be2de7e6..0288cbc4 100644
--- a/examples/gmail_xoauth.phps
+++ b/examples/gmail_xoauth.phps
@@ -5,6 +5,7 @@
 
 //Import PHPMailer classes into the global namespace
 use PHPMailer\PHPMailer\PHPMailer;
+use PHPMailer\PHPMailer\SMTP;
 use PHPMailer\PHPMailer\OAuth;
 
 // Alias the League Google OAuth2 provider class
@@ -25,10 +26,10 @@ $mail = new PHPMailer;
 $mail->isSMTP();
 
 //Enable SMTP debugging
-// 0 = off (for production use)
-// 1 = client messages
-// 2 = client and server messages
-$mail->SMTPDebug = 2;
+// SMTP::DEBUG_OFF = off (for production use)
+// SMTP::DEBUG_CLIENT = client messages
+// SMTP::DEBUG_SERVER = client and server messages
+$mail->SMTPDebug = SMTP::DEBUG_SERVER;
 
 //Set the hostname of the mail server
 $mail->Host = 'smtp.gmail.com';
diff --git a/examples/pop_before_smtp.phps b/examples/pop_before_smtp.phps
index 94222d81..31d7876a 100644
--- a/examples/pop_before_smtp.phps
+++ b/examples/pop_before_smtp.phps
@@ -8,6 +8,7 @@
 use PHPMailer\PHPMailer\PHPMailer;
 use PHPMailer\PHPMailer\Exception;
 use PHPMailer\PHPMailer\POP3;
+use PHPMailer\PHPMailer\SMTP;
 
 require '../vendor/autoload.php';
 
@@ -22,10 +23,10 @@ $mail = new PHPMailer(true);
 try {
     $mail->isSMTP();
     //Enable SMTP debugging
-    // 0 = off (for production use)
-    // 1 = client messages
-    // 2 = client and server messages
-    $mail->SMTPDebug = 2;
+    // SMTP::DEBUG_OFF = off (for production use)
+    // SMTP::DEBUG_CLIENT = client messages
+    // SMTP::DEBUG_SERVER = client and server messages
+    $mail->SMTPDebug = SMTP::DEBUG_SERVER;
     //Set the hostname of the mail server
     $mail->Host = 'mail.example.com';
     //Set the SMTP port number - likely to be 25, 465 or 587
diff --git a/examples/smtp.phps b/examples/smtp.phps
index 792d8d9c..97a42d3a 100644
--- a/examples/smtp.phps
+++ b/examples/smtp.phps
@@ -5,6 +5,7 @@
 
 //Import the PHPMailer class into the global namespace
 use PHPMailer\PHPMailer\PHPMailer;
+use PHPMailer\PHPMailer\SMTP;
 
 //SMTP needs accurate times, and the PHP time zone MUST be set
 //This should be done in your php.ini, but this is how to do it if you don't have access to that
@@ -17,10 +18,10 @@ $mail = new PHPMailer;
 //Tell PHPMailer to use SMTP
 $mail->isSMTP();
 //Enable SMTP debugging
-// 0 = off (for production use)
-// 1 = client messages
-// 2 = client and server messages
-$mail->SMTPDebug = 2;
+// SMTP::DEBUG_OFF = off (for production use)
+// SMTP::DEBUG_CLIENT = client messages
+// SMTP::DEBUG_SERVER = client and server messages
+$mail->SMTPDebug = SMTP::DEBUG_SERVER;
 //Set the hostname of the mail server
 $mail->Host = 'mail.example.com';
 //Set the SMTP port number - likely to be 25, 465 or 587
diff --git a/examples/smtp_no_auth.phps b/examples/smtp_no_auth.phps
index c635434e..6b3fc7d1 100644
--- a/examples/smtp_no_auth.phps
+++ b/examples/smtp_no_auth.phps
@@ -5,6 +5,7 @@
 
 //Import the PHPMailer class into the global namespace
 use PHPMailer\PHPMailer\PHPMailer;
+use PHPMailer\PHPMailer\SMTP;
 
 //SMTP needs accurate times, and the PHP time zone MUST be set
 //This should be done in your php.ini, but this is how to do it if you don't have access to that
@@ -17,10 +18,10 @@ $mail = new PHPMailer;
 //Tell PHPMailer to use SMTP
 $mail->isSMTP();
 //Enable SMTP debugging
-// 0 = off (for production use)
-// 1 = client messages
-// 2 = client and server messages
-$mail->SMTPDebug = 2;
+// SMTP::DEBUG_OFF = off (for production use)
+// SMTP::DEBUG_CLIENT = client messages
+// SMTP::DEBUG_SERVER = client and server messages
+$mail->SMTPDebug = SMTP::DEBUG_SERVER;
 //Set the hostname of the mail server
 $mail->Host = 'mail.example.com';
 //Set the SMTP port number - likely to be 25, 465 or 587
diff --git a/examples/ssl_options.phps b/examples/ssl_options.phps
index 674079b6..6c697155 100644
--- a/examples/ssl_options.phps
+++ b/examples/ssl_options.phps
@@ -5,6 +5,7 @@
 
 //Import the PHPMailer class into the global namespace
 use PHPMailer\PHPMailer\PHPMailer;
+use PHPMailer\PHPMailer\SMTP;
 
 //SMTP needs accurate times, and the PHP time zone MUST be set
 //This should be done in your php.ini, but this is how to do it if you don't have access to that
@@ -19,10 +20,10 @@ $mail = new PHPMailer;
 $mail->isSMTP();
 
 //Enable SMTP debugging
-// 0 = off (for production use)
-// 1 = client messages
-// 2 = client and server messages
-$mail->SMTPDebug = 2;
+// SMTP::DEBUG_OFF = off (for production use)
+// SMTP::DEBUG_CLIENT = client messages
+// SMTP::DEBUG_SERVER = client and server messages
+$mail->SMTPDebug = SMTP::DEBUG_SERVER;
 
 //Set the hostname of the mail server
 $mail->Host = 'smtp.example.com';
diff --git a/src/PHPMailer.php b/src/PHPMailer.php
index 978e00ee..367daf7e 100644
--- a/src/PHPMailer.php
+++ b/src/PHPMailer.php
@@ -3,7 +3,7 @@
  * PHPMailer - PHP email creation and transport class.
  * PHP Version 5.5.
  *
- * @see       https://github.com/PHPMailer/PHPMailer/ The PHPMailer GitHub project
+ * @see https://github.com/PHPMailer/PHPMailer/ The PHPMailer GitHub project
  *
  * @author    Marcus Bointon (Synchro/coolbru) <phpmailer@synchromedia.co.uk>
  * @author    Jim Jagielski (jimjag) <jimjag@gmail.com>
@@ -23,10 +23,10 @@ namespace PHPMailer\PHPMailer;
 /**
  * PHPMailer - PHP email creation and transport class.
  *
- * @author  Marcus Bointon (Synchro/coolbru) <phpmailer@synchromedia.co.uk>
- * @author  Jim Jagielski (jimjag) <jimjag@gmail.com>
- * @author  Andy Prevost (codeworxtech) <codeworxtech@users.sourceforge.net>
- * @author  Brent R. Matzelle (original founder)
+ * @author Marcus Bointon (Synchro/coolbru) <phpmailer@synchromedia.co.uk>
+ * @author Jim Jagielski (jimjag) <jimjag@gmail.com>
+ * @author Andy Prevost (codeworxtech) <codeworxtech@users.sourceforge.net>
+ * @author Brent R. Matzelle (original founder)
  */
 class PHPMailer
 {
@@ -388,11 +388,11 @@ class PHPMailer
      * SMTP class debug output mode.
      * Debug output level.
      * Options:
-     * * `0` No output
-     * * `1` Commands
-     * * `2` Data and commands
-     * * `3` As 2 plus connection status
-     * * `4` Low-level data output.
+     * * SMTP::DEBUG_OFF: No output
+     * * SMTP::DEBUG_CLIENT: Client messages
+     * * SMTP::DEBUG_SERVER: Client and server messages
+     * * SMTP::DEBUG_CONNECTION: As SERVER plus connection status
+     * * SMTP::DEBUG_LOWLEVEL: Noisy, low-level data output, rarely needed
      *
      * @see SMTP::$do_debug
      *
@@ -1109,9 +1109,11 @@ class PHPMailer
     protected function addAnAddress($kind, $address, $name = '')
     {
         if (!in_array($kind, ['to', 'cc', 'bcc', 'Reply-To'])) {
-            $error_message = sprintf('%s: %s',
+            $error_message = sprintf(
+                '%s: %s',
                 $this->lang('Invalid recipient kind'),
-                $kind);
+                $kind
+            );
             $this->setError($error_message);
             $this->edebug($error_message);
             if ($this->exceptions) {
@@ -1121,10 +1123,12 @@ class PHPMailer
             return false;
         }
         if (!static::validateAddress($address)) {
-            $error_message = sprintf('%s (%s): %s',
+            $error_message = sprintf(
+                '%s (%s): %s',
                 $this->lang('invalid_address'),
                 $kind,
-                $address);
+                $address
+            );
             $this->setError($error_message);
             $this->edebug($error_message);
             if ($this->exceptions) {
@@ -1157,7 +1161,7 @@ class PHPMailer
      * Uses the imap_rfc822_parse_adrlist function if the IMAP extension is available.
      * Note that quotes in the name part are removed.
      *
-     * @see    http://www.andrew.cmu.edu/user/agreen1/testing/mrbs/web/Mail/RFC822.php A more careful implementation
+     * @see http://www.andrew.cmu.edu/user/agreen1/testing/mrbs/web/Mail/RFC822.php A more careful implementation
      *
      * @param string $addrstr The address list string
      * @param bool   $useimap Whether to use the IMAP extension to parse the list
@@ -1227,12 +1231,15 @@ class PHPMailer
         $name = trim(preg_replace('/[\r\n]+/', '', $name)); //Strip breaks and trim
         // Don't validate now addresses with IDN. Will be done in send().
         $pos = strrpos($address, '@');
-        if (false === $pos or
-            (!$this->has8bitChars(substr($address, ++$pos)) or !static::idnSupported()) and
-            !static::validateAddress($address)) {
-            $error_message = sprintf('%s (From): %s',
+        if (false === $pos 
+            or (!$this->has8bitChars(substr($address, ++$pos)) or !static::idnSupported()) 
+            and !static::validateAddress($address)
+        ) {
+            $error_message = sprintf(
+                '%s (From): %s',
                 $this->lang('invalid_address'),
-                $address);
+                $address
+            );
             $this->setError($error_message);
             $this->edebug($error_message);
             if ($this->exceptions) {
@@ -1368,7 +1375,7 @@ class PHPMailer
      * - Conversion to punycode is impossible (e.g. required PHP functions are not available)
      *   or fails for any reason (e.g. domain contains characters not allowed in an IDN).
      *
-     * @see    PHPMailer::$CharSet
+     * @see PHPMailer::$CharSet
      *
      * @param string $address The email address to convert
      *
@@ -1434,8 +1441,8 @@ class PHPMailer
      */
     public function preSend()
     {
-        if ('smtp' == $this->Mailer or
-            ('mail' == $this->Mailer and stripos(PHP_OS, 'WIN') === 0)
+        if ('smtp' == $this->Mailer
+            or ('mail' == $this->Mailer and stripos(PHP_OS, 'WIN') === 0)
         ) {
             //SMTP mandates RFC-compliant line endings
             //and it's also used with mail() on Windows
@@ -1482,10 +1489,12 @@ class PHPMailer
                 }
                 $this->$address_kind = $this->punyencodeAddress($this->$address_kind);
                 if (!static::validateAddress($this->$address_kind)) {
-                    $error_message = sprintf('%s (%s): %s',
+                    $error_message = sprintf(
+                        '%s (%s): %s',
                         $this->lang('invalid_address'),
                         $address_kind,
-                        $this->$address_kind);
+                        $this->$address_kind
+                    );
                     $this->setError($error_message);
                     $this->edebug($error_message);
                     if ($this->exceptions) {
@@ -1602,7 +1611,7 @@ class PHPMailer
     /**
      * Send mail using the $Sendmail program.
      *
-     * @see    PHPMailer::$Sendmail
+     * @see PHPMailer::$Sendmail
      *
      * @param string $header The message headers
      * @param string $body   The message body
@@ -1732,7 +1741,7 @@ class PHPMailer
     /**
      * Send mail using the PHP mail() function.
      *
-     * @see    http://www.php.net/manual/en/book.mail.php
+     * @see http://www.php.net/manual/en/book.mail.php
      *
      * @param string $header The message headers
      * @param string $body   The message body
@@ -2178,8 +2187,8 @@ class PHPMailer
         }
 
         return $this->encodeHeader($this->secureHeader($addr[1]), 'phrase') . ' <' . $this->secureHeader(
-                $addr[0]
-            ) . '>';
+            $addr[0]
+        ) . '>';
     }
 
     /**
@@ -4198,7 +4207,7 @@ class PHPMailer
      * Multi-byte-safe pathinfo replacement.
      * Drop-in replacement for pathinfo(), but multibyte- and cross-platform-safe.
      *
-     * @see    http://www.php.net/manual/en/function.pathinfo.php#107461
+     * @see http://www.php.net/manual/en/function.pathinfo.php#107461
      *
      * @param string     $path    A filename or path, does not need to exist as a file
      * @param int|string $options Either a PATHINFO_* constant,
@@ -4405,7 +4414,7 @@ class PHPMailer
      * Uses the 'relaxed' algorithm from RFC6376 section 3.4.2.
      * Canonicalized headers should *always* use CRLF, regardless of mailer setting.
      *
-     * @see    https://tools.ietf.org/html/rfc6376#section-3.4.2
+     * @see https://tools.ietf.org/html/rfc6376#section-3.4.2
      *
      * @param string $signHeader Header
      *
@@ -4449,7 +4458,7 @@ class PHPMailer
      * Uses the 'simple' algorithm from RFC6376 section 3.4.3.
      * Canonicalized bodies should *always* use CRLF, regardless of mailer setting.
      *
-     * @see    https://tools.ietf.org/html/rfc6376#section-3.4.3
+     * @see https://tools.ietf.org/html/rfc6376#section-3.4.3
      *
      * @param string $body Message Body
      *
diff --git a/src/POP3.php b/src/POP3.php
index 66cf2730..51de2b32 100644
--- a/src/POP3.php
+++ b/src/POP3.php
@@ -3,7 +3,7 @@
  * PHPMailer POP-Before-SMTP Authentication Class.
  * PHP Version 5.5.
  *
- * @see       https://github.com/PHPMailer/PHPMailer/ The PHPMailer GitHub project
+ * @see https://github.com/PHPMailer/PHPMailer/ The PHPMailer GitHub project
  *
  * @author    Marcus Bointon (Synchro/coolbru) <phpmailer@synchromedia.co.uk>
  * @author    Jim Jagielski (jimjag) <jimjag@gmail.com>
@@ -29,14 +29,14 @@ namespace PHPMailer\PHPMailer;
  *   and then loop through your mail sending script. Providing this process doesn't
  *   take longer than the verification period lasts on your POP3 server, you should be fine.
  * 3) This is really ancient technology; you should only need to use it to talk to very old systems.
- * 4) This POP3 class is deliberately lightweight and incomplete, and implements just
+ * 4) This POP3 class is deliberately lightweight and incomplete, implementing just
  *   enough to do authentication.
  *   If you want a more complete class there are other POP3 classes for PHP available.
  *
- * @author  Richard Davey (original author) <rich@corephp.co.uk>
- * @author  Marcus Bointon (Synchro/coolbru) <phpmailer@synchromedia.co.uk>
- * @author  Jim Jagielski (jimjag) <jimjag@gmail.com>
- * @author  Andy Prevost (codeworxtech) <codeworxtech@users.sourceforge.net>
+ * @author Richard Davey (original author) <rich@corephp.co.uk>
+ * @author Marcus Bointon (Synchro/coolbru) <phpmailer@synchromedia.co.uk>
+ * @author Jim Jagielski (jimjag) <jimjag@gmail.com>
+ * @author Andy Prevost (codeworxtech) <codeworxtech@users.sourceforge.net>
  */
 class POP3
 {
diff --git a/src/SMTP.php b/src/SMTP.php
index e0f42de7..71978b36 100644
--- a/src/SMTP.php
+++ b/src/SMTP.php
@@ -24,8 +24,8 @@ namespace PHPMailer\PHPMailer;
  * PHPMailer RFC821 SMTP email transport class.
  * Implements RFC 821 SMTP commands and provides some utility methods for sending mail to an SMTP server.
  *
- * @author  Chris Ryan
- * @author  Marcus Bointon <phpmailer@synchromedia.co.uk>
+ * @author Chris Ryan
+ * @author Marcus Bointon <phpmailer@synchromedia.co.uk>
  */
 class SMTP
 {
@@ -59,26 +59,36 @@ class SMTP
 
     /**
      * Debug level for no output.
+     *
+     * @var int
      */
     const DEBUG_OFF = 0;
 
     /**
      * Debug level to show client -> server messages.
+     *
+     * @var int
      */
     const DEBUG_CLIENT = 1;
 
     /**
      * Debug level to show client -> server and server -> client messages.
+     *
+     * @var int
      */
     const DEBUG_SERVER = 2;
 
     /**
      * Debug level to show connection status, client -> server and server -> client messages.
+     *
+     * @var int
      */
     const DEBUG_CONNECTION = 3;
 
     /**
      * Debug level to show all messages.
+     *
+     * @var int
      */
     const DEBUG_LOWLEVEL = 4;
 
@@ -745,7 +755,7 @@ class SMTP
      *
      * @return bool
      *
-     * @see    hello()
+     * @see hello()
      */
     protected function sendHello($hello, $host)
     {
@@ -880,10 +890,10 @@ class SMTP
         }
 
         return $this->sendCommand(
-           'RCPT TO',
-           $rcpt,
-           [250, 251]
-       );
+            'RCPT TO',
+            $rcpt,
+            [250, 251]
+        );
     }
 
     /**