Re: I think I speak for everybody when I say...
"What is so offensive about such a letter?"
To you - you say you'd laugh about it if it had been sent to you - it's probably not offensive.
I, too, would laugh about it. I wouldn't personally find it offensive if I was the recipient. (Though I would be certain to put it online for all to see).
However, it wasn't either of us that received it. It was an elderly couple who, and I quote, "were more than a little upset to be addressed in this manner"
That's what makes it offensive - it offended the couple it was sent to.