To get to the trailhead, take I-90 to exit 34, turn left back under I-90 and follow 486th Ave SE until it ends in a T-instersection. Take a right onto SE Middle Fork Road and follow it, staying left at the Lake Dorothy road fork. Proceed just a short distance until you see a clear cut hill on the right side of the road, with a yellow gate blocking a logging road heading up the hill. Park across the road from the gate, and walk up the gated logging road, keeping to the right at an intersection. Follow this road, keeping a sharp eye on the left side of the road. About 100' before the road passes over a culvert, you might spy a black and blue Crest toothbrush sticking out of the ground. That's the start of the trail! Walk for a ways through second growth forest, and along side the first large stream you come to. Do not cross this stream! The trail will become more obvious as you veer to the left, and start heading up hill. And continue heading up hill. And up. And up! This trail doesn't believe in switchbacks, so you gain your elevation in a fairly minimal distance. You'll probably want to bring your ice axe for the descent - it's muddy and slick in several places, and steep everywhere!
At times the track gets faint, but if you continue working your way up the ridge line in general, you won't be too far off. Eventually, the path leaves the forest, and breaks out to what would be meadows in the spring and summer. We encountered 2-3 feet of snow from here to the summit which was just enough to cover the underbrush, but not enough to support your weight. This presented us with many opportunities to post hole up to our thighs, and we took advantages of several of them! The most difficult area to ascend, because of the snow, was a rock slide that you come to just before the final summit ridge. There was enough snow here to make it difficult to see your footing, and to make what footing you found quite slick. The rocks are also not entirely stable, and choosing a path through and over this, without being able to see what you were placing your foot on, took quite a bit of care and concentration. On the way down, we talked to a gentleman doing some trail maintenance (thanks, Vic!), who told us that if you work your way around on the left side of the rock slide, there's a route that goes up through some trees, and lets you avoid the rock slide entirely. That might reduce the possibility of a twisted ankle or barked shin.
Once you're above the rock slide, the summit is just a ridge stroll away - but it's still a steep ridge! Head up towards the obvious rock outcropping, then just a few hundred feet more to the actual summit. I've heard that there's a mailbox on the summit (thus the peak's name), filled with amusing reading material to peruse (including a copy of "Green Eggs and Ham" for the Green Team from the 1996 Basic class). However, on this day, the mailbox was snugly buried beneath several feet of snow, so the reading material will have to await another ascent, sometime after the snow is gone.
A side note: A large portion of Mailbox Peak is scheduled to be logged soon. If you want to walk through second growth forests rather than clear cuts, do this climb sooner rather than later. Also, the more traffic this trail sees, the more the company logging the mountain (Weyerhauser, I think) will be pressured into repairing the trail after the logging is done.