Apple Mail - Junk mail filters aren't working in Mail on Mac

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Mail for Mac junk mail settings.

This article applies to Mac OS Mojave 10.14. Settings for Mac OS High Sierra 10.13 may be slightly different.

If the Mail app on your Mac incorrectly identifies junk mail, try these suggestions.

If junk mail isn’t marked as junk
If junk mail isn’t highlighted in brown to indicate it’s junk, check your junk mail preferences.

Choose Mail > Preferences, then click Junk Mail. Make sure “Enable junk mail filtering” and “Mark as junk mail, but leave it in my Inbox” are selected.
If you chose the junk mail preference to exempt addresses in the Previous Recipients list from filtering, check that the list contains only valid addresses you want to receive messages from. See Delete email addresses.
If the problem persists, click Reset in Junk Mail preferences to reset the junk mail database to its original information. Resetting the database removes everything Mail has learned from you about what is junk or not junk, so you need to help Mail identify junk mail again.

Be sure to tell Mail when it misses junk mail: Select the message, then click the Junk button in the Mail toolbar (or use the Touch Bar).

If emails are incorrectly marked as junk
There are several ways you can prevent Mail from incorrectly marking legitimate messages as junk.

Tell Mail that a message is legitimate by marking it as not junk. Click Not Junk in the banner of the message; or select the message, then click the Not Junk button in the Mail toolbar (or use the Touch Bar).
Add the sender to the Contacts app. See Use information found in Mail in other apps.
Review your junk mail settings in the Junk Mail pane of Mail preferences, as well as any rules you might have created to handle junk mail, and make changes as necessary.