Error wpMandrill login lockout alert

Checking our error.log file I'm seeing a lot of the following. (note i replace our actual domain with "mydomain.com"):

[Mon Mar 06 14:43:12.629613 2017] [:error] [pid 8943] [client 97.74.195.196:53057] \nwpMandrill::wp_mail_native: webmaster@mydomain.com (Login lockout alert for http://mydomain.com/)\n

Can you tell me what's causing this? We use Mandrill as our email relay and the only errors that we are seeing are the ones related to "Login lockout alert for ..." which seems to be related to Defender.