Thanks for your reply.
We did the same by going to virustotal.com and uploading only wp-mailinglist-plugin.php of the latest lite version and the paid version and the result is that only one detected something, this one: Bkav Pro – VEX.Webshell
We’re looking to see if it’s a false positive (as it should be as we never meddle with our own files in any illicit way, ever).
One of our staff says that it is only a Vietnamese scanner that detects something (Bkav Pro) and none of the big known names do. See this also: https://www.alfredforum.com/topic/16478-virus-total-detection/
And, it’s a false positive. Use tools like this to confirm that it is clean: https://shellray.com/
We have our own tools in our own hosting company and none detect that and we use our own plugin for our Newsletters. It is a false positive by Bkav Pro.
You can keep using our plugin safely. If anything is discovered, we’ll patch it immediately.
I’m interested in seeing what tool your hosting used, so that we can verify that.