I did a google search and came up with this:
https://www.wired.com/story/how-to-passcode-lock-any-app-on-your-phone/
You have a couple of other tools you can turn to in iOS: They weren't primarily intended for securing apps, but they can do the same job. The first is Screen Time, which you can access from Settings: If you tap Use Screen Time Passcode to set a passcode, then select App Limits and set the daily limit for an app to zero hours zero minutes, you're effectively locking other people out of the app without the passcode.
I just looked and under the Productivity and Finance category is where Mail shows up. Based on the above it seems to work - but I couldn't set the time to 0 hours 0 minutes - I had to use a minimum of 1 minute. But it did work.