Water in the letterbox, why?

Yes I absolutely KWYM!!

Thanks Greyduster we do have a glass panel above it so maybe thats it.. I think I'll find the number for the guy who put it in and ask him what he suggests.. OH wants to use clear mastic to seal things but he is a bit heavy handed and I don't want to spoil the look of the door IFYKWIM

We had a similar problem after our door had been in for less than a year. We got the supplier back and he checked everything out, including the glass seals, and said he couldn't work out why it was getting in, but that the manufacturer did not guarantee door furniture, which included the letterbox! As we had put quite a lot of work his way over the years, he fitted a new, slightly different, letterbox and didn't charge us, and we didn't have the problem again. Have you got glass panels above the letter box? Could it be leaking in through those? The problem with water is it's insidious. It will always find a way in somewhere. We had the door fitter back again early this year as water was pouring down the inside of the door when the rain was heavy and the wind was in the wrong direction. He resealed it inside and out - and it still got in. It was finding its way in under the tiles on the porch into some boxing over the inside of the door. Not much help I know, but I hope you get it sorted soon.