How to block the domain appspot.com

It is overwhelming. I am getting hundreds of spam messages daily from ...appspot.com.
I have tried blocking the domain in the email client as well as adding them to the Pihole blacklist but they keep coming. I am at a loss on what to do next. Does anyone have any suggestions?

Are you getting them via email?

Yes. They go directly into my junk folder using free Outlook.

Thank you for responding.

Pi-hole blocks domains on DNS, its not going to block emails. You need to see if you mail provider can delete those mails directly instead of just moving them to the spam.

Pi-hole is not the right tool for you in this case. Sorry.

Understood. Thank you for responding.