Response from Support:
We are aware of this issue and based on the limited internal discussion about this yesterday that I’ve seen, I expect this will have to be urgently prioritised and a fix / update created and published as soon as possible.
Given that the plugin is not overly complex hopefully this will be done soon.
The work around for now is to use the SMTP settings instead of the plugin to use SparkPost with WordPress.
The plug-in is just an interface to send mail. You can just as easily use the SMTP interface and standard SMTP settings in WordPress.
