well I hae had a bit of a think about this letter, and decided that since 1. It has never happened before 2. My youngest son just got engaged (and he will be at the sedding)
I will treat it like an error - that is, despite the evidence to the contrary, that this organisation made a genuine mistake in sending it to me, and email my xh to tell him that I have a letter here, and does he wish me to bin it or forward it?
Why not? I am not angry with him any more. Taking the line 'these things happen' is non passive aggressive, and as I will have to meet him at my son's wedding next year, I might as well be superficially friendly.
I honestly have no desire to open it, bin it without his say so, or return to sender. If it happens again, that is another matter.
For me, at present, behaving as if it is a error on someone'e part seems like the best option, and I am being uninvolved, and slightly helpful..