No one was hurt and damage was minor.
The pillbox was thrown by a group of men at around 3:15 a.m., police said.
Police officials ruled out the involvement of a terror group.
No suspects have been named, but they suggested that the blast may be linked to growing anger at the Yuchengco Group of Companies over the failure of its pre-need education plan Pacific Plans Inc. The family also controls Grepalife.
Meanwhile, PPI condemned the bombing at Grepalife, saying it was part of an orchestrated attack to prevent the company from defending itself and availing of remedies under the law.
"We urge the perpetrators to abide by the law and respect the legal process and not to resort to violence and illegal means," the company said in a statement. — Michael Punongbayan, Cecille Suerte Felipe