Invalid Firebase path: .com. Firebase paths must not contain '.', '#', '$', '[', or ']'

As could be noticed from the error, Firebase path(key) doesn't allow certain special characters in the path.
While storing email as path, I recommend you to encode the email i.e., replace 'dots' from 'commas'. If you wan't to retrieve, you can decode.

public static String EncodeString(String string) {
    return string.replace(".", ",");
}

public static String DecodeString(String string) {
    return string.replace(",", ".");
}

Let me know, how it goes for you.

Update

Using UIDs to store user details is better than Email IDs.