Email not being received, but "Test email has been sent." message given

Trying to use send test email produces notification "Test email has been sent." but no email is ever received at the address provided.

Also seen are the following notifications:

Notice: Undefined index: id in Drupal\mandrill\Plugin\Mail\MandrillMail->mail() (line 98 of modules/custom/mandrill/src/Plugin/Mail/MandrillMail.php).
...
Notice: Undefined index: id in Drupal\mandrill\Plugin\Mail\MandrillMail->mail() (line 98 of modules/custom/mandrill/src/Plugin/Mail/MandrillMail.php).
...
Notice: Undefined index: body in Drupal\mandrill\Plugin\Mail\MandrillMail->mail() (line 115 of modules/custom/mandrill/src/Plugin/Mail/MandrillMail.php).
...
Notice: Undefined index: id in Drupal\mandrill\Plugin\Mail\MandrillMail->mail() (line 140 of modules/custom/mandrill/src/Plugin/Mail/MandrillMail.php).
...
Notice: Undefined index: id in Drupal\mandrill\Plugin\Mail\MandrillMail->mail() (line 188 of modules/custom/mandrill/src/Plugin/Mail/MandrillMail.php).
...
Notice: Undefined index: module in Drupal\mandrill\Plugin\Mail\MandrillMail->mail() (line 189 of modules/custom/mandrill/src/Plugin/Mail/MandrillMail.php).
...
Notice: Undefined index: id in Drupal\mandrill\Plugin\Mail\MandrillMail->mail() (line 203 of modules/custom/mandrill/src/Plugin/Mail/MandrillMail.php).

Also, Mandrill settings shows this message persistently:

Notice: Undefined property: GuzzleHttp\Psr7\Response::$code in Drupal\mandrill\DrupalMandrill->call() (line 106 of modules/custom/mandrill/src/DrupalMandrill.php).
Drupal\mandrill\DrupalMandrill->call('subaccounts/list', Array)
Mandrill_Subaccounts->getList()
Drupal\mandrill\MandrillAPI->getSubAccounts()
Drupal\mandrill\Form\MandrillAdminSettingsForm->buildForm(Array, Object)
call_user_func_array(Array, Array)
Drupal\Core\Form\FormBuilder->retrieveForm('mandrill_admin_settings', Object)
Drupal\Core\Form\FormBuilder->buildForm(Object, Object)
Drupal\Core\Controller\FormController->getContentResult(Object, Object)
call_user_func_array(Array, Array)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
Drupal\Core\Render\Renderer->executeInRenderContext(Object, Object)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->wrapControllerExecutionInRenderContext(Array, Array)
Drupal\Core\EventSubscriber\EarlyRenderingControllerWrapperSubscriber->Drupal\Core\EventSubscriber\{closure}()
call_user_func_array(Object, Array)
Symfony\Component\HttpKernel\HttpKernel->handleRaw(Object, 1)
Symfony\Component\HttpKernel\HttpKernel->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\Session->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\KernelPreHandle->handle(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->pass(Object, 1, 1)
Drupal\page_cache\StackMiddleware\PageCache->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\ReverseProxyMiddleware->handle(Object, 1, 1)
Drupal\Core\StackMiddleware\NegotiationMiddleware->handle(Object, 1, 1)
Stack\StackedHttpKernel->handle(Object, 1, 1)
Drupal\Core\DrupalKernel->handle(Object)