What makes a serve legal?

Under Law 2.6:

1. Start with the ball resting freely on the open palm of your stationary free hand.
2. Project it near vertically upwards, without spin, so it rises at least 16 cm after leaving your palm.
3. Strike it as it is falling, without it touching anything first.
4. In singles it must bounce on your court first, then directly on the receiver's court (it can go anywhere on the receiver's side). In doubles it goes right half-court to right half-court.
5. From the start of the serve until you strike it, the ball must be above the level of the playing surface and behind your end line.
6. The ball must not be hidden from the receiver by you, your doubles partner or anything you wear or carry.
7. As soon as you toss the ball, move your free arm and hand out of the space between the ball and the net.

It is your responsibility to serve so the umpire can see it is legal. On the first doubtful serve in a match, the umpire may warn you; after that, any serve by you or your partner that is not clearly legal is a fault. The umpire can relax these requirements for a player whose physical disability prevents compliance.