Tests are good but moving left by lifting invariants into the type system is better.

Compare

   fn send_email(addr: &str, subject: &str, body: &str) -> Result<()>

to

    fn send_email(add: &EmailAddr, subject: &str, body: &str) -> Result<()>

In the second case, the edge cases of an empty or invalid email address don't need to be tested, they are statically impossible.