Swiftmailer unable to authenticate to GMail using oauth2 token - php

Question

I'm trying to use SwiftMailer to log in to a GMail account for which I have a valid access token. I can successfully login using the account credentials:
$Transport = new Swift_SmtpTransport('smtp.gmail.com',587,'tls');
$Transport->setAuthMode('login')
->setUsername('my-email-address')
->setPassword('my-password');
$Mailer = new Swift_Mailer($Transport);
// ... make a new message & send it
However if I change the code to use my oauth2 token like so:
$access_token = 'ya29.GLv....';
$Transport = new Swift_SmtpTransport('smtp.gmail.com',587,'tls');
$Transport->setAuthMode('XOAUTH2')
->setUsername('my-email-address')
->setPassword($access_token);
$Mailer = new Swift_Mailer($Transport);
I get an error message: Swift_TransportException: Expected response code 250 but got code "535".... Username and Password are not accepted.
I'm using the same access token elsewhere using the GMail API, so I know the token is valid.
What am I missing?
Edit
Just ran the code again with the debugger turned on. It looks like the first error code thrown is 334 with the message:
Expected response code 235 but got code "334", with message "334 eyJzdGF0dXMiOiI0MDAiLCJzY2hlbWVzIjoiQmVhcmVyIiwic2NvcGUiOiJodHRwczovL21haWwuZ29vZ2xlLmNvbS8ifQ==
The encoded part of that message decodes to:
{"status":"400","schemes":"Bearer","scope":"https://mail.google.com/"}

Answer 1

The biggest hurdle that you might face and no one is talking about is insufficient permissions. SwiftMailer doesn't do a good job elaborating this issue; therefore you might want to use GoogleClient to make sure you credentials are working before using them with SwiftMailer.
Full source code
# Using SwiftMailer
// Create the Transport
$transport = (new Swift_SmtpTransport('smtp.gmail.com', 587, 'tls'))
->setAuthMode('XOAUTH2')
->setUsername('<SENDER_EMAIL>')
->setPassword('<ACCESS_TOKEN>');
// Create the Mailer using your created Transport
$mailer = new Swift_Mailer($transport);
// Create a message
$message = (new Swift_Message())
->setFrom(['<SENDER_EMAIL>' => '<SENDER_NAME>'])
->setTo(explode(';', '<RECEIVER_EMAIL>'))
->setSubject('<SUBJECT')
->setBody('<MESSGAE_BODY>');
// Send the message
if ($mailer->send($message)) {
print 'Mail sent...';
exit();
}
echo('Mail not sent...');