The three-vehicle crash happened at about 2:20 a.m. on northbound Highway 880 just north of Broadway, CHP Officer Robert Haven said.
The mail truck, a big rig, burst into flames and blocked all northbound lanes for about three hours, according to the CHP.
At least one person suffered moderate injuries, Haven said.
The two left lanes of northbound Highway 880 were reopened at about 5:20 a.m. A third lane was reopened at around 8:15 a.m. and the fourth lane was reopened at 8:45 a.m..
The truck was not carrying any mail when it crashed, CHP Officer Kevin Kroncke said.