Fix all the issues reported by phpcs

I will make this issue broader, since phpcs reports also when a class doesn't inject its dependencies.

+  /**
+   *
+   */
   public static function create(ContainerInterface $container) {

A documentation comment for a method defined in an interface should not be empty.

+  /**
    * Creates a new \Drupal\graph_mail\Plugin\Mail\GraphMail object.
    */
-  public function __construct(array $configuration, $plugin_id, $plugin_definition, LoggerChannelFactoryInterface $logger) {
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, ConfigFactoryInterface $configFactory, LoggerChannelFactoryInterface $logger) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
     $this->logger = $logger;
+    $this->config = $configFactory;
+
   }

When adding new parameter to a method, those parameters need to be added at the end. Also, the code should be backward compatible.

-      $guzzle = new \GuzzleHttp\Client();
+      $guzzle = new Client();

That class is used to implement a service.

Thank you for working on this!

+  /**
+   * {@inheritdoc}
+   */
   public static function create(ContainerInterface $container) {
     return new static(
       $container->get('config.factory'),
-      $container->get('language_manager')
+      $container->get('language_manager'),
+      $container->get('plugin.manager.mail'),
+      $container->get('logger.factory')
     );
   }

Since the code uses only a single logger channel, there is no need to store the logger factory. Only the logger channel is necessary.

-  public function __construct(ConfigFactoryInterface $config_factory, LanguageManagerInterface $language_manager) {
+  public function __construct(ConfigFactoryInterface $config_factory, LanguageManagerInterface $language_manager, MailManagerInterface $mail_manager, LoggerChannelFactoryInterface $logger) {
     parent::__construct($config_factory);
     $this->languageManager = $language_manager;
+    $this->mailManager = $mail_manager;
+    $this->logger = $logger;
   }

When a dependency is added, the code should be similar to the following one, used by Drupal core in GDToolkit::__construct().

public function __construct(array $configuration, $plugin_id, array $plugin_definition, ImageToolkitOperationManagerInterface $operation_manager, LoggerInterface $logger, ConfigFactoryInterface $config_factory, StreamWrapperManagerInterface $stream_wrapper_manager, FileSystemInterface $file_system = NULL) {
  parent::__construct($configuration, $plugin_id, $plugin_definition, $operation_manager, $logger, $config_factory);
  $this->streamWrapperManager = $stream_wrapper_manager;
  if (!$file_system) {
    @trigger_error('The file_system service must be passed to GDToolkit::__construct(), it is required before Drupal 9.0.0. See https://www.drupal.org/node/3006851.', E_USER_DEPRECATED);
    $file_system = \Drupal::service('file_system');
  }
  $this->fileSystem = $file_system;
}

(I am referring to what that code does with $file_system, which is a parameter with a default value.)

-  public function __construct(array $configuration, $plugin_id, $plugin_definition, LoggerChannelFactoryInterface $logger) {
+  public function __construct(array $configuration, $plugin_id, $plugin_definition, LoggerChannelFactoryInterface $logger, ConfigFactoryInterface $config_factory) {
+    parent::__construct($configuration, $plugin_id, $plugin_definition);
     $this->logger = $logger;
+    $this->config = $config_factory;
+
   }

Since StringTranslationTrait is used, the constructor needs to also call setStringTranslation() passing the translation service, which will then need to be one of the constructor's parameters.
There should not be any empty line before the closing curly parenthesis.

-      $message = t('Something went wrong: @error', ['@error' => $e->getMessage()]);
-      \Drupal::logger('graph_mail')->log($message, 'error');
+      $message = $this->t('Something went wrong: @error', ['@error' => $e->getMessage()]);
+      $this->logger->get('graph_mail')->log($message, 'error');

The first argument of log() must be a literal string, not the output of $this->t() or t(). I would also use code similar to the code shown for watchdog_exception(). (I won't call watchdog_exception(), as that is going to be deprecated.)

Before fixing the issues reported by phpcs, I will implement some other changes that will change or reduce the required dependencies.