Yes, I'm fairly sure my office has an outgoing-mail box which I could use if I wanted to. I probably would be willing to use it for actual letters I'd written and stamped, if it weren't for the fact that I think it's in the other building and hence actually further out of my way than the postbox I (hope I) go past on my way to buy lunch.

I'm not sure I'd be as comfortable with dropping things into that which were labelled "not known at this address". Somehow I feel as if I should take the responsibility myself of putting things like that into an actual postbox. I'm not entirely sure why. I think it's a combination of the idea that if it doesn't successfully get back to the Royal Mail the resulting inconvenience is borne by somebody else rather than me (giving me a greater responsibility to personally make sure it does), and the fact that it hasn't got a stamp on it in the normal sense (and so somebody checking through the work outbox might have to stop and think about it and perhaps reject it on grounds of excessive weirdness).