Why was that a certified cheque though? That's literally like sending 300k in cash... by mail.

I typically see those used like when buying a car off Kijiji or Facebook where you can't really trust a random person's check to go through or trust the seller to give you the car when the cheque goes through. Hand over a certified cheque, you're safe to deposit it and hand over the keys.

I don't see how that kind of guarantee was needed to deposit an inheritance cheque. There's plenty of legal ways to approach this if it bounces or whatever.