Did you know that in Germany, for example, many email providers are considered telecommunications services? This means that authorities can request your inventory data and emails. But even without official surveillance, emails are exposed to many dangers. An email passes through many nodes on its way to a mailbox and can be read at numerous points. In addition, numerous freemail services such as GMX.de or WEB.de probably use the content of your emails to show you targeted advertising.
Unfortunately, it is not always possible to encrypt emails because, for example, the other party does not offer encryption. If encryption is not possible, switch to secure messenger apps such as Signal.