DKIM error when sending mail with GSuite using PHP Mailer - php

Question

I am hosted at A2 hosting, but I am using GSuite to handle all my mail.
When I send test message to mail-tester.com from Gmail I get an awesome rating.
However when I sent a message using my PHP script:
$mail = new PHPMailer(true);
ob_start();
//Server settings
$mail->SMTPDebug = SMTP::DEBUG_SERVER; // Enable verbose debug output
$mail->Host = 'smtp.gmail.com'; // Set the SMTP server to send through
$mail->SMTPAuth = true; // Enable SMTP authentication
$mail->Username = $pickuploc . '#xxxx.com'; // SMTP username
$mail->Password = 'xxx';
$mail->SMTPSecure = PHPMailer::ENCRYPTION_STARTTLS; // Enable TLS encryption; `PHPMailer::ENCRYPTION_SMTPS` also accepted
$mail->Port = 587; // TCP port to connect to
//Recipients
$mail->setFrom($pickuploc . '#xxx.com', 'xxx xxxx');
$mail->addAddress($email, $fname . " " . $lname); // Add a recipient
$mail->addBCC($pickuploc . '#xxx.com');
// Content
$mail->isHTML(true); // Set email format to HTML
include 'email-confirmed.html';
$mail->Subject = 'Your Reservation Has Been Confirmed!';
$mail->Body = ob_get_clean();
$mail->AltBody = 'Your reservation has been confirmed.';
$mail->send();
I get an error from mail-tester.com that reads my DKIM is not valid.
I think this is because I am sending form a foreign server (not Google) and my MX records point to Google, but I really need these emails to get though, how should I work this problem?
Is there a way to configure this in PHP Mailer? Thanks.

Answer 1

The reason it's complaining is because you're not signing with DKIM at all!
You can do DKIM signing with PHPMailer, but there's a certain amount of setup required. PHPMailer provides some code to help you do this. Make sure you're using PHPMailer 6.1.1 or later; older versions have bugs affecting DKIM signing.
First of all you need to create your DKIM keys and put them in your DNS.
Now you need to alter your script to sign messages using your private key, as shown in this example; the parts you need to add are:
//This should be the same as the domain of your From address
$mail->DKIM_domain = 'example.com';
//See the DKIM_gen_keys.phps script for making a key pair -
//here we assume you've already done that.
//Path to your private key:
$mail->DKIM_private = 'dkim_private.pem';
//Set this to your own selector
$mail->DKIM_selector = 'phpmailer';
//Put your private key's passphrase in here if it has one
$mail->DKIM_passphrase = '';
//The identity you're signing as - usually your From address
$mail->DKIM_identity = $mail->From;
//Suppress listing signed header fields in signature, defaults to true for debugging purpose
$mail->DKIM_copyHeaderFields = false;
//Optionally you can add extra headers for signing to meet special requirements
$mail->DKIM_extraHeaders = ['List-Unsubscribe', 'List-Help'];