Async inside onCustomValidation

Is it possible to use async await within onCustomValidation() function?

In order to prevent duplications in my DB, I used onCustomValidation function that calls a function from the backend which gets the input value, checks if it already exists in the DB and if so - rejects.
But even though the value exists, I still don’t manage to prevent it from being inserted into my DB (as far as the function concerns, it is a valid input).

This is the code in the Home module (front):

 // Check if email value already exists in the DB
  $w("#emailInput").onCustomValidation( async function (value, reject){
    $w("#errorMessage").text = "Please enter a valid email address";
    let found = false;
    found = await findEmail(value).then( (exist) =>{
      if(exist) {
        console.log("Email address already exists");
        $w("#errorMessage").text = "Email address already exists!"
        return true;
    } );
    console.log("found: " + found);
    if(found) { reject("Email address already exists"); }
  } );

And this is the code from the backend module:

// Returns true if and only if email is already exists in FullFunnel collection
export function findEmail (email) 
 return wixData.query("FullFunnel").eq("email", email).find().then
    (results) => {
      if (results.items.length > 0) {  
        return true;
      return false;
  ).catch(err => console.log(err));

Any ideas?