If you're sending an e-invoice to an Estonian company and you're not sure if they can receive e-invoices, then you can check it from Estonian E-Business Register.
Just search using your customers business name and check whether they can or cannot receive e-invoices.
If your customer can or cannot receive e-invoices it is clearly stated under Receipt of invoices section.
If you want to send e-invoices to a Finnish customer, then you can check it from verkkolaskuosoite.fi.