Why are exports included in estimation of domestic product by expenditure method? 
or
Is export a part of domestic product? 

Yes, because all the goods and services which are exported are produced by the producers in the domestic economy. For instance, Indian tea, coffee, jute goods, etc. which are purchased by foreigners are produced in India and it is called India's export. In short, since exported goods and services are produced in domestic territory of a country, therefore, export of goods and services is a part of gross domestic product (GDP). Export receipts are not 'net factor income from abroad' as they are revenue of the firms from sale of their products.