This plugin was installed and not activated in my wp-content/plugins folder.
Two days later I received a message from my host saying the ‘wp-responder-email-autoresponder-and-newsletter” folder contained malicious scripts. The folder was quarantined and I was advised to remove it from the host and my local machine.
A scan with AVG revealed two emails with viruses received via pop3 on my local machine the same day as the folder was installed. I haven’t received an email virus in over 1 year. (Might be a coincidence, but worth noting.)