I was involved in correspondence with a union rep and used her firm's address and the owner of the firm opened the letter
and I have to say I couldnt get anyone interested in that at all. The Royal Mail said there responsibility stopped at putting it in the letterbox
however some else said correctly that there is extensive legislation about interfering with communication electronic and otherwise
Even the Union rep bwubbled her lips and Protested in the Strongest Terms - and then did nothing ! and went on bwubbling her lips
But blimey it did teach me not to write incriminating letters to common addresses - I mean there was NOTHING in it -luckily for all parties concerned. Oh and I also have started sending letters to X's address , addressed to Y and then refer (within the law of libel) to |X's extreme gullibility in believing such and such, or how easy so and so found it to pull the wool over his eyes. Par for the course - I know these letters are opened now, but getting them to be believed is the trick, I think.