Playing devils advocate... if a government would use this technique then the government would know which person got which address. So they would know afterwards which person voted a certain way. In certain countries you would get in trouble from this point if you voted for opposition.

Next thing would be that nobody knows if every address was really sent to a real person. For example you add a certain amount of addresses for old users and government could raise the amount of votes for ruling parties because they could keep those addresses and vote themself. It would be risky to steal votes because even if government would be sure that a person that got an address sent, is dead, you never know that someone could find out that dead were voting. Or how to ensure that really the real receivers of the address are voting. There were cases were letter voting cards from workers of a local company were used by the company to vote for the local companies boss's daugther so she gets in local parliament. To have power to make decisions for the company again.

So i think the part from votes till checking if votes are correctly counted are fine. Only how to know that the votes are valid are a bit difficult. Ok, nowadays offline votes arent perfect in that matter too. You cant really say that the real voters voted by letter, for example. They lower the risk a bit by counting the votes in small areas first and then send the results to the bigger parts of the votes. So citizens of the small area could see if the votes are strange. More votes than voters or completely different votes than the people would have voted.
The thing most hackers and pirate parties fear the most is the might of the one operating the voting process. Centralized data... and making wrong things with them.