Lauren Weinstein on Nostr: Email in many respects is highly fungible, especially for spam and phishing. So ...

Email in many respects is highly fungible, especially for spam and phishing. So senders move rapidly by routing their emails through an ever-changing list of (usually insecure) servers, scattered around the world. They may use one address/server for a day or just an hour before they move on. Much spam even comes from major email providers -- Gmail has become a major source of phishing emails sent to destinations outside Gmail. They just keep moving, and the per email cost is essentially nil.

That's the short answer.