Would you be able to provide test cases?

Thank you for the fun Kata!

I agree with tom-lord 100%. I know you said not to overcomplicate it but for 5kyu it should be more thorough. More examples of why the test are too easy is would be the following.

tes.t.ema.il@email.com = valid
test..email@email.com = invalid

or

test."email".test@email.com = valid
test"email"test@email.com = invalid

yet like tom-lord explained
"testemail"@email.com = valid

you could even use an email address with an IP address for the domain part.
testemail@[127.127.127.127] = valid
testemail@[IPv6:2001:db8::1] = valid

I know the last part is rare but it still is an email address.