preparedoauthcredentials.php

См. документацию.

<?php
 
namespace Bitrix\Main\Mail\Smtp;

class PreparedOauthCredentials extends \PHPMailer\PHPMailer\OAuth
{
    public function __construct(array $options)
    {
        parent::__construct($options);
        $this->oauthToken = $options['token'];
    }

    public function getOauth64(): string
    {
        return base64_encode(
            'user=' .
            $this->oauthUserEmail .
            "\001auth=Bearer " .
            $this->oauthToken .
            "\001\001"
        );
    }
}