[TYPO3-mvc] filter for frontend user in isValid validator

[Domi]

Hello,

I want to search for duplicated frontendUser inside of my isValid custom 
validator of powermail.

I tried following:

$user = 
$this->frontendUserRepository->findByUsername('test at test.de')->count();

There is a user with Username test at test.de in the database. Not sure why 
the result is always 0, do someone have a hint? I dont want to write or 
copy the Repositories and Models, I only need to check if the user 
exists in the database.

Can somebody help me out?

Thanks,
Dominic