Gmail is a powerful tool for communications that uses machine learning to optimize services but there are some things you can do to make it better
Mark incorrectly classified messages as Not spam
Gmail uses machine learning to better understand your preferences, and to recognize spam. When you manually mark a message as not spam, Gmail learns how to manage messages addressed to you.
Add trusted senders to your personal contacts list
Users in your domain can manage their personal contacts list in Gmail. When a user adds an external address to their contacts list, Gmail won't mark messages from the external address as spam.
Google also offers support articles to help you set filters
https://support.google.com/mail/answer/6579?hl=en
You can also take a few minutes to follow along with this in-depth YouTube tutorial