First check that it is indeed water. Then realize that the dryer moves water in the from of vapor out through the duct, so condensation can form inside the duct.
Also, and this could be what is happening here, the outside of the duct and other parts inside the dyer can have condensation form on the outside, if the humidity in the room (common in laundry areas and in general) is high enough and outside temperature cool enough. Often this is because cool air is drawn back in through the dryer duct, either from convection/stack effect or via use of other exhaust fans in the home.
The exit of the vent should be inspected to ensure the back-draft flap is working, and to make sure lint isn't clogging things up. Having the exit under the porch is less than ideal, it would be better to use rigid duct and extend it to where it can be inspected, and to keep the moisture from damaging the porch structure. Also, less likely that rodents will enter.
