Answer
Give the environment's Default Mail Sender a distinct, identifiable email address (something that clearly marks it as coming from the non-production environment) and enable the 'Override Mail Sender' setting so that address is used regardless of what an individual event/user would otherwise specify. Then, working with your mail server/IT team, add a relay rule on the mail server itself that forwards everything sent from that specific address to whatever distribution list or admin mailbox should actually receive it. This combination reliably intercepts all event-triggered mail from that environment without needing to touch every individual event action.