Banned Words in Incoming Emails
The Admin > Site Settings page contains a way to block incoming emails that contain certain words or phrases.
This is a blunt instrument. Think carefully about how this could block legitimate email.
Do not put the words "virus" or "malware" or "spam" in this list. Nearly every email in the world will contain headers that include those words, indicating the email has been checked for viruses, malware, and spam.