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 = \PHPMailer\PHPMailer\SMTP::DEBUG_SERVER;
$mail->SMTPDebug = 2; //Alternative to above constant
$mail->isSMTP(); // tell the class to use SMTP
$mail->SMTPAuth   = true;                // enable SMTP authentication
$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

Debug levels

Setting the PHPMailer->SMTPDebug property to these numbers or constants (defined in the SMTP class) results in different amounts of output:

• SMTP::DEBUG_OFF (0): Disable debugging (you can also leave this out completely, 0 is the default).
• SMTP::DEBUG_CLIENT (1): Output messages sent by the client.
• SMTP::DEBUG_SERVER (2): as 1, plus responses received from the server (this is the most useful setting).
• SMTP::DEBUG_CONNECTION (3): as 2, plus more information about the initial connection - this level can help diagnose STARTTLS failures.
• SMTP::DEBUG_LOWLEVEL (4): as 3, plus even lower-level information, very verbose, don’t use for debugging SMTP, only low-level problems.

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 takes 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 as complex as you like - for example you could capture all the output and store it in a database.

And finally, don’t forget to disable debugging before going into production.