Once your one or more email addresses are sold on the dark web, they give raise to a S..t storm of spam and unless you use a spam filter like SpamSieve, or set up a rule set based on the Return-path header address, you won't stop or control the deluge.
The Return-path header field contains the string associated with the spammer's account on either their personally controlled email server, or on a commercial email relay server. Sometimes, the spammer will force this to be blank on their own servers. Multiple spam campaigns can be run from the same server by simply adding another account on that server. Spammers can then use entirely different From and Subject header fields to vary their campaign.
In Apple Mail, select just one of these spam messages and then visit Mail > Settings > Rules. This is important to build a Return-path rule.
When you Add a new rule, you will see the following default where the right-field will be the From header from your selected spam message. In this example, the Apple email was not spam:
When you click on that From selector, there is a long list of options, but no Return-path entry. At the bottom of that drop down list is the Edit header list… menu item. Select that.
Add [ + ] the Return-path header name to the list and click OK.
Now, when you click that From selector in the Mail rule, you will see Return-path on that long list of options:
Choose Return-path, and the Return-path string from your previously selected spam Mail message will automatically be inserted in the right-most field of that rule. Somewhere in the middle of that long string, is a domain name that is not yours, or Amazons, but might be something like "nonsense@mylife.com" or "annoyances@foocareers.net" You just need to remove everything in the Return-path string except the domain name (e.g. mylife.com, foocareers.net).
Buried in the Return-Path string is a domain name that one can base the Mail rule upon. I simply remove all of the irrelevant parts of that server string using arrow keys and backspace except the offending domain name and end up with a rule line that can manage one or more spam emails. At the end of the rule panel, I have:
[ Move Message ↕︎ ] to mailbox: [ Trash ↕︎ ]
When I save the rule set, a dialog appears offering to apply the rule to your inbox and any messages (selected or not) that have a matching Return-path content you edited above, and these will be removed. Future inbound spam emails with that particular matching Return-path string in your saved rule will be processed before they hit your inbox.
You can clean up your Trash and/or Junk from the Apple Mail > Mail menu with two obvious menu items.
