Why not "if this then that" to delete known spam?

Like many folks here, I get an average of 10 fake “Google” and “FedEx” messages every day. As others have noted, the return address is not even remotely related to either of those companies. Ninetyfive percent of them are in my Junk box with yellow-ish text. Would it impede SpamSieve to set up a simple rule such as “If message content includes ‘fedex.com’ and return address does not contain ‘fedex.com’ then delete the message”?

That’s fine so long as you put it above the SpamSieve rule.

However, if the messages are in the junk mailbox with yellow text, that suggests that they were moved there by a server junk filter. This is because SpamSieve normally uses the Spam mailbox, and Mail’s junk filter should be off. If the messages are going to the Junk mailbox because of a server filter, that happens before they get to your Mac, so they will not be seen by Mail’s rules, so your rule won’t do anything (unless you make a server rule).

Is there any way to turn off that server junk filter in Apple Mail or create a server rule that would do the same? “Enable junk mail filtering” is not selected in Mail prefs, and SpamSieve is the first rule.

The setting in Mail only affects Mail’s client junk filter. Typically you can configure a server filter by logging into the Web site for your mail provider. For some examples, please see this page.