Is it legal to be in a foreign country and mail your passport back to US for visa to another country?

Question: We're in Thailand and need to apply for an Indian business visa. Is it legal to mail our passports back to the US and apply for an Indian business

Source: http://www.us-passport-service-guide.com/is-it-legal-to-be-in-a-foreign-country-and-mail-your-passport-back-to-us-for-visa-to-another-country.html

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