Duh!
Yes and no. As you probably know, as a result of cutbacks etc, there are more cut off times in collection of the mail. If you posted something early in the day, the chances are yes, but at some boxes, the last collection is actually the morning one, so in practice, something posted after it has been collected can sit there till the next collection.
Also. the journey has got to add something, so there is probably an earlier cut off time to get what mail there is across so you can have a normal delivery, in which case anything that arrived after would miss it.