The client who wouldn't open his post

An accountant told me about one of his early clients who, years ago, never opened any official looking envelopes. 

The accountant had to do this when he visited each month. And he then told the client what action needed to be taken in each case. 

 In an effort to get the client to open letters from the accountant himself he started sending them in distinctive grey coloured envelopes. However the client rarely opening these either. 

On one occasion the accountant wrote: "I'm writing this letter knowing that I'll be the one to open it when I come to visit you next month." He laughed when that is exactly what happened.

On another occasion the accountant sent his bill in a pretty coloured envelope. The accompanying letter started: "I hope you like the envelope. I used it in the hope that you'll have opened it without realising it was from me!"