The VirusTotal API key is necessary to use this add-on. This is because email attachments are uploaded to them in order to have them scanned. To find out more, please take a look at the following article: here.
In order to fetch the results from VirusTotal after the attachments have been uploaded, it is necessary to continuously poll their server. This is because the time to scan each add-on is not constant and varies (based on the size as well as the current amount of requests received). Because of this, it may be necessary to update the parameters below to achieve better results.