How to stop emails going out to customers from non prod environments

Summary:

We want a way to prevent any emails going out to customers and suppliers by any means in our lower non prod environments. I have seen few posts in customer connect but i am unable to locate them now. Our first option is to disable at SMTP level so that no external emails are sent from non prod Cloud erp environments. If that is not feasible out next option is at a setup and maintenance level to turn off external emails (if such a setting do exist) We tried data masking but that mased all data including person