Would you like to be sure that only the correct recipient has access to the attachments? Have them authenticate via SMS.
With the introduction of the GDPR, extra security measures for personal data have become the new standard. You want to send your e-mail so that it is not only encrypted but so that it can only be opened by the intended recipient. Via Two Factor Authentication (2FA) or SMS Authentication you can protect your e-mail by sending the recipient an SMS with a one-time login code or password. This way you can be sure that only the authorised recipient gains access to the e-mail.