It's an example of a poorly written email, it looks like somebody is angry and yelling at you, these types of emails can be described as a poor etiquette.
Answer:
Assuming an upstander is someone that opposes cyberbullying:
(1) Call them out
(2) Report the behavior to proper moderation authority
(3) Tell the person to block messages from the cyberbully
I don't really know what else you want from this.
Cheers.
