In France, letterboxes in doors are very much the exception rather than the rule. Like most people, we have instead a boite postale (postbox). Commonly, these are stuck on top of a pole at the side of the road. However, ours – a metal one, as per the approved design – is actually built in to a stone gate post.
So my question is: given its secure and watertight location, – and the fact that it’s open for an average of no more than five seconds a day, six days a week – how in blue blazes did that snail get inside it? It’s not the first either. Do snails have access to technology that would put Star Trek to shame?

How big is the slot to put the mail into the box? Not big enough for a snail to slip through? Or is it solid, and the mailman has a key? Perhaps it was in the stack of mail before the mailman put it into your box. One of life’s mysteries.
Well, this is what I don’t understand. The postie has a key to put in larger packages, but if it’s just a letter it’s necessary to raise quite a heavy (especially if you happen to be a snail with the encumbrance of a shell) flap and insert the envelope through a very narrow gap. I don’t think it’s a question of hitching a ride on a parcel, because this has happened more than once.
You raise an excellent question. SnailGate!