However if the address exist and you send btc mistakenly to that address, you will never get back it.
Addresses are just numbers and we encode them in base58check to prevent typos. Whatever software you use will only check whether the checksum is valid or not. It won't, and can't, make sure that somebody somewhere has access to the corresponding private key.
I assume when DomainMagnate says the address "doesn't exist" he means that the checksum doesn't match and therefore the address is "invalid".
Then again he is a sig ad spammer, so he might just be making up nonsense and hoping that it sounds like a "quality post" so he can get paid.
I had to clarify because it's a common misconception. People then build on that and wonder whether their wallet has to be online for them to receive bitcoin. Whether they'll lose money if they are not online. You see what I'm talking about?