Google Gmail offers the option to send a confidential email. The recipient will need to verify their identity to read the email, and you can also automatically have the email expire at a time of your choosing.
Video tutorial:
Click on Compose.
On a Windows Desktop, click on the Toggle confidential mode icon.
For Android and iOS devices, click on the three dots in the top-right corner and click Confidential mode.
Set an expiration date from 1 day to 5 years.
Choose whether to require a passcode. If you don't use a passcode, the email will expire at your set time, and sending an email is the same otherwise from here.
If you decided to use a passcode, click on SMS passcode, then OK.
Compose your email normally by entering a recipient, subject, and email body.
Click on Send when your email is ready.
Enter a phone number, including the area code, to send an SMS (Text message) to the recipient so they can open the email.
When the recipient opens the email, they will be notified that it is a confidential email, and they can click a link to view the email.
Once they click to view the email, they can click on SEND PASSCODE to get the passcode to open the email.
Once the recipient enters the SMS code, they can view the email.
You can remove access at any time by finding the sent email in your Sent box.