The email is encrypted in transit using TLS. It is then unencrypted and re-encrypted (by us) for storage on our servers using zero-access encryption. Once zero-access encryption has been applied, no-one except you can access emails stored on our servers (including us). It is not end-to-end encrypted, however, and might be accessible to the sender’s email service.
Here is a reference from ProtonMail: https://proton.me/support/proton-mail-encryption-explained