Known accounts are classified as junk. How stop

In Apple Mail, you have two options:

(1) If the junk mail is being incorrectly identified, you can go to Apple Mail Preferences, select Junk Mail, and disable (turn off) junk mail filtering.

(2) Leave the junk mail filtering on but for messages that are not junk but are incorrectly flagged as junk, manually correct each one ("move to In box"). Eventually Apple Mail will "learn" not to flag those as junk.

You have other options in those Apple Mail Junk preferences. For instance, you can check the box to not mark messages from your Contacts list as junk. That should stop "known accounts" being flagged as junk. Also you can tell Mail to not flag messages from previous recipients as junk, etc. Check the other options there also.

To add to good advise already posted - who is the Mail Server attached to the e-mail Account ? Is the Mail Server marking / identifying the e-mail as Junk even before they arrive to you mail box. That could also come into play.