If it was in the UK: The company must have an official address. You look up that address at companies house.gov.uk. Any mail delivered to that address is legally received by the company; if they don't read it or act on it then it is their problem. You can send your resignation in a printed letter using recorded delivery, then whatever happens, the company has received it legally. What could go wrong?
The company's address might be wrong. That's their problem, using registered mail the letter will return with a note "not at this address". They might refuse to take a letter, the registered mail goes back to you, and it's their fault. They might ignore the letter, that's their fault.
Of course that's not the usual way to hand in your notice. It's the way to do it if you try to hand in your notice and nobody replies. The best result is always peacefully parting your ways. The second best result is parting your ways, and if the company complains about anything, you can prove that it is their fault. And if your manager doesn't respond, HR would be the next to contact - just in case your manager is in hospital after a car crash, for example.