Earlier quoted context omitted.
>If you send a letter to an address of course you are consenting for the postman to read your mail. It's more like if the person you are sending to hires an assistant to sort and organize their mail for them (which people actually do.) You then go and sue the assistant.
If the assistant read confidential mail that they have not been approved to do, yes, they can get sued for that. Many people sign NDA's. If you have an assistant read mails covered by the NDA, then the party who sent the mail can sue. and so on and so on.
I'm not sure how common it is, and it's probably less so now with email, but I'd guess a lot of important people do have their assistants screen their confidential mail. For example, I know politicians often have people help with their mail.