Most rich email clients (Outlook, Apple, Office 365, Gmail or Mozilla Thunderbird) allow users and developers to automate tasks based on incoming or outgoing emails. Actions allowed by some of the email clients are far more sophisticated than others. Almost all email clients allow automatic email forwarding or blocking/deleting future incoming emails based on something in the targeted email. In most cases the user doesnt even have to open up the email; it only needs to arrive in the users inbox to activate the rule.”]
Source: https://www.csoonline.com/article/3431425/beware-rogue-email-rules-and-forms.html