Is it possible to use mutt (neomutt) email clinet with nextcloud?

Is there a port or interface for isync or other method so I can use mutt(neomutt) email client?
What is the internal storage system for emails?

No, such a port doesn’t exist. By default Nextcloud doesn’t store any email, except you’ve installed the Mail app. In this case email information will be stored in database tables.