Issue #3324340: Added the configuration file for the default values

config/install/graph_mail.settings.yml

+tenant_id: ''
+client_id: ''
+client_secret: ''
+user_id: ''
+version: '1.0'
+default_mail: ''

src/Form/MailSettingsForm.php

@@ -97,7 +97,7 @@ public function buildForm(array $form, FormStateInterface $form_state): array {
 
     $form['default_mail'] = [
       '#title' => $this->t('Default mail'),
-      '#description' => $this->t('Set the mail used to send emails. By default, the site mail is used.'),
+      '#description' => $this->t('The mail used to send emails. Leave it empty to use the site mail.'),
       '#type' => 'textfield',
       '#default_value' => $config->get('default_mail'),
     ];