Is it correct if i am using await + ToListAsync() over IQueryable which is not defined as a task

At the beginning I thought I will get an error because I am using "await" a method which is not defined as a task, but the above worked well

Actually, you are awaiting a method which returns a Task<T>, where T is a List<TSet>. If you look at the extension method QueryableExtensions.ToListAsync, you'll see that it returns a Task<List<TSource>>. You are asynchronously waiting on this method to query the database, create the list and return it back to the caller. When you await on such a method, the method won't return until the operation has completed. async-await makes your code feel synchronous, while execution is actually asynchronous.

Actually there is no problem because you are awating the ToListAsync() not the getAllScanEmailTo().

EDIT: To see how async-await pattern is working you can see this link. Here is a usefull image from there

You are not "awaiting a method". You are awaiting a Task, which is an awaitable.

You call getAllScanEmailTo that returns an IQueryable<TSet> on which you then call ToListAsync which returns the Task<List<TSet>> you are awaiting.