Building an email client with Python/Django

I want to build an email client webApp using Django/Python, so that Outlook users can connect to it and send/receive emails.

I have used MSAL (the Microsoft identity platform endpoint in order to authenticate users and access secured web APIs) to get token with the OAuth standard. In order to store users emails and sync them, I have thought of storing the Token in server-side and create a process that runs every 15 min to get updates from outlook users accounts (new received email, new sent email ...) and store them in MongoDB Atlas.

I don't think this is the good way to do it since the token have to be on client side. Can anyone please suggest a more secure and efficient way to do it ?

Thank you very much !