How do I stop a email sender from automatically going to the junk folder

Do you now have any rules set that would send that user or subject matter or some other attribute would sent it to Junk?

Have you went to the webmail and see if any Rules are set there?

From the subject matter or body content does it see like Junk? Maybe the email provider is categorizing it as Junk.

Try setting a Rule in Mail Preferences>Rules to sent mails messages to an inbox if it has the sender listed. This may not work if the email provider called it Junk before it reached your Mail app.