added a form validation function to make sure entered email matches the chosens orgs email address

ill_corporate_forms.module

@@ -12,6 +12,7 @@ use Drupal\Core\Form\FormStateInterface;
  */
 function ill_corporate_forms_form_node_ill_institution_request_form_alter(array &$form, FormStateInterface $form_state, string $form_id): void {
   _ill_corporate_forms_add_item_type_states($form);
+  $form['#validate'][] = '_ill_corporate_forms_validate_partner_email';
 }
 
 /**
@@ -19,6 +20,7 @@ function ill_corporate_forms_form_node_ill_institution_request_form_alter(array
  */
 function ill_corporate_forms_form_node_ill_institution_request_edit_form_alter(array &$form, FormStateInterface $form_state, string $form_id): void {
   _ill_corporate_forms_add_item_type_states($form);
+  $form['#validate'][] = '_ill_corporate_forms_validate_partner_email';
 }
 
 /**
@@ -88,3 +90,44 @@ function _ill_corporate_forms_add_item_type_states(array &$form): void {
   }
 }
 
+/**
+ * Validates that the submitted email matches the partner organization's ILL email.
+ */
+function _ill_corporate_forms_validate_partner_email(array &$form, FormStateInterface $form_state): void {
+  $partner_value = $form_state->getValue('field_ill_partner_organization');
+  $email_value = $form_state->getValue('field_ill_req_email');
+
+  // Extract the partner node ID.
+  $partner_nid = NULL;
+  if (is_array($partner_value) && isset($partner_value[0]['target_id'])) {
+    $partner_nid = $partner_value[0]['target_id'];
+  }
+
+  // Extract the submitted email.
+  $submitted_email = '';
+  if (is_array($email_value) && isset($email_value[0]['value'])) {
+    $submitted_email = $email_value[0]['value'];
+  }
+  elseif (is_string($email_value)) {
+    $submitted_email = $email_value;
+  }
+
+  if (empty($partner_nid) || empty($submitted_email)) {
+    $form_state->setErrorByName('field_ill_partner_organization',
+      t('There is an error with the selected Partner Org, It may not be configured correctly.'));
+    return;
+  }
+
+  // Load the partner institution and compare emails.
+  $partner_node = \Drupal::entityTypeManager()->getStorage('node')->load($partner_nid);
+  if (!$partner_node || $partner_node->bundle() !== 'ill_institution') {
+    return;
+  }
+
+  $partner_email = $partner_node->get('field_ill_email')->value;
+  if (!empty($partner_email) && mb_strtolower($submitted_email) !== mb_strtolower($partner_email)) {
+    $form_state->setErrorByName('field_ill_req_email',
+      t('The email address entered does not match the ILL email on file for the selected partner organization.'));
+  }
+}
+