Solukhumbu District is one of 14 districts of Koshi Province of eastern Nepal. As the name suggests, it consists of the subregions Solu and Khumbu. The closest post office to Solukhumbu with a postal code assigned to it is the Sindhuli D.P.O., which has the postal code 56000.
Wikipedia

Nepal officially the Federal Democratic Republic of Nepal is a landlocked country in South Asia located mainly in the Himalayas but also includes parts of the Indo-Gangetic Plain. With an estimated population of 26.4 million, it is 48th largest country by population and 93rd largest country by area. It borders China in the north and India in the south, east, and west while Bangladesh is located within only 27 km (17 mi) of its southeastern tip and Bhutan is separated from it by the Indian state of Sikkim.

Wikipedia