On entry of test-email, 'Unexpected error: ... Drupal\Component\Plugin\Exception\PluginNotFoundExceptio"

with module/smtp composer-installed on Drupal 8.0.3,

@

admin/config/system/smtp

On edit in the UI, entry of any address

Send test e-mail
E-mail address to send a test e-mail to
[                                                    ]
Type in an address to have a test e-mail sent there.

Results is

"The website encountered an unexpected error. Please try again later.

Drupal\Component\Plugin\Exception\PluginNotFoundException: The "" plugin does not exist. in Drupal\Core\Plugin\DefaultPluginManager->doGetDefinition() (line 57 of core/lib/Drupal/Component/Plugin/Discovery/DiscoveryTrait.php)."