Update Note_for_SMTP_debugging.txt

docs/Note_for_SMTP_debugging.txt

@@ -1,17 +1,44 @@
+#SMTP Debugging
+
 If you are having problems connecting or sending emails through your SMTP server, the SMTP class can provide more information about the processing/errors taking place.
 Use the debug functionality of the class to see what's going on in your connections. To do that, set the debug level in your script. For example:
 
-$mail->SMTPDebug = 1;
-$mail->isSMTP();  // telling the class to use SMTP
+```php
+$mail->SMTPDebug = 2;
+$mail->isSMTP();  // tell the class to use SMTP
 $mail->SMTPAuth   = true;                // enable SMTP authentication
-$mail->Port       = 26;                  // set the SMTP port
+$mail->Port       = 25;                  // set the SMTP port
 $mail->Host       = "mail.yourhost.com"; // SMTP server
 $mail->Username   = "name@yourhost.com"; // SMTP account username
 $mail->Password   = "your password";     // SMTP account password
+```
 
-Notes on this:
-$mail->SMTPDebug = 0; ... will disable debugging (you can also leave this out completely, 0 is the default)
-$mail->SMTPDebug = 1; ... will echo errors and server responses
-$mail->SMTPDebug = 2; ... will echo errors, server responses and client messages
+##Debug levels
+
+Setting the `SMTPDebug` property results in different amounts of output:
+
+`0`: Disable debugging (you can also leave this out completely, 0 is the default).
+`1`: Output messages sent by the client.
+`2`: as 1, plus responses received from the server (this is probably the most useful setting for debugging).
+`3`: as 2, plus more information about the initial connection.
+`4`: as 3, plus even lower-level information, very verbose.
+
+You don't need to use levels above 2 unless you're having trouble connecting at all - it will just make output more verbose and more difficult to read.
+
+Note that you will get no output until you call `send()`, because no SMTP conversation takes place until you do that.
+
+##Debug output format
+
+The form that the debug output taks is determined by the `Debugoutput` property. This has several options:
+
+ * `echo` Output plain-text as-is, appropriate for CLI
+ * `html` Output escaped, line breaks converted to `<br>`, appropriate for browser output
+ * `error_log` Output to error log as configured in php.ini
+
+By default PHPMailer will use `echo` if run from a `cli` or `cli-server` SAPI, `html` otherwise. Alternatively, you can implement your own system by providing a callable expecting two parameters: a message string and the debug level:
+
+    $mail->Debugoutput = function($str, $level) {echo "debug level $level; message: $str";};
+
+You can of course make this more complex - for example your could capture all the output and store it in a database.
 
 And finally, don't forget to disable debugging before going into production.