I don't quite agree with that, Doll.
"Postmen wear caps" suggests that you mean 'all postmen' -- i.e. no postmen are omitted.
If you say 'the postmen' instead, that is not necessarily a reference to 'all postmen everywhere', but rather it is a reference to a specific group and in that limited group, all of the postmen wear caps. In other words, there could be postmen who do not wear caps who are not part of the group referred to.