I have a vent pipe that runs horizontally through the attic and then turns down for a vent in the eave. Sometimes I get some condensation dripping from the fan.  I think/thought it is because of the humidity outside and the fact that the moist air has to be pushed down when it gets to the eave and it struggles to escape. I've always thought about running it vertical and through the roof someday to avoid this. Maybe I'm wrong?????