That's BS. Get a different maintenance man. He don't know anything. It doesn't matter if it's a bathroom sink. If the water drains out you need a different sized gasket. It's that simple. You see, like other leak-stopping areas of your sink, the plug, or stopper, can get gunked up or its gasket can wear out which causes a drain. You can fix this without much trouble. You won't even need to turn off the water to the sink, although you should make sure no one turns on the faucet while you're working. For this job, you'll need channel-type pliers, a small wire brush and a screwdriver. The control mechanism for a pop-up type stopper is a lever-type assembly called the clevis. It's a vertical rod that extends down from the faucet area and attaches to a horizontal pivot rod, which juts from a retaining nut located on the drain pipe between the drain opening and the P-trap (the big bend in the pipe). The pivot rod is attached to the bottom part of the stopper. When you engage the stopper by pulling up on the clevis, it pulls the pivot rod up and out, yanking the stopper down into place and holding it there. Go ahead and engage the stopper. Shine a light on the horizontal pivot rod and look for the retaining nut that holds the rod in place. Unscrew the nut and pull out the rod. Go back to the top of the sink and pull the stopper up out of the drain completely. Clean away gunk with a wire brush. If the stopper has a gasket, replace the gasket with one the same size. Put the stopper back into the sink, lining up the small eye hole on the bottom end with the pivot rod. Reinsert the pivot rod and tighten the retaining nut. Test the stopper to ensure it works and holds water. If the stopper doesn't go all the way down, or the clevis won't stay up after you engage it, adjust the position of the clevis on the pivot rod. Loosen the small screw that attaches the clevis to the pivot rod and slide the clevis up or down the rod. Retighten and test. If it still leaks then replace the stopper and the the faucet which comes with it's own stopper. But bathroom sinks are meant to hold water if all the parts are correct. Your plumber just handed you a line of bs since it was clear he really didn't know what was causing the drainage.