The answer is it depends really on the email provider you are using, and the email provider your recipient is using on the other end.
In general, email is encrypted “in transit” if the sending and receiving email service providers both support transport layer security (TLS). This enables you to secure data in transit (i.e., when it moves from one party to the other).
In order for emails to be encrypted fully before being sent — meaning that they’re still unreadable to unauthorized users once they’re sitting on the recipient’s email server — both parties must use the same email encryption service (such as ProtonMail).
Examples of fully expected services (between individuals on the service) include things like: