If your sending it to work your making it their business too. So you cant complain too much if they open it. Its not there job to handle your personal mail so their doing that free for you.
However its funny how much views differ on this.
Some people seem to think opening someone else mail is akin to murder and should be punished as so.
Where as, its never occurred to me it was particularly important. Most mail is just junk anyhow.
When I’ve lived in shared houses you obviously get sent mail for previous tenants. Some of the current tenants have thought opening their mail was a seriously breach of privacy. Where as I thought if there still getting mail sent to their old house it cant be too important, is probably junk and its the previous tenants fault for not getting their address changed.
Therefore I always open it to check its not something important before generally chucking/recycling it. Once it was a letter from some baliffs for the old tenant, which to be fair to the baliffs was quickly sorted, but if we hadnt opened it we’d have never known.