How is a customer email set to a quote?

Way 1: using the \Magento\Quote\Model\Quote::setCustomerEmail() magic method:

1.1. Magento\Checkout\Model\Type\Onepage::_prepareGuestQuote()

1.2. Magento\Quote\Model\QuoteManagement::placeOrder()

1.3. Magento\Persistent\Model\QuoteManager::setGuest()

1.4. Magento\Braintree\Model\Paypal\Helper\OrderPlace::prepareGuestQuote()

1.5. Magento\Paypal\Model\Express\Checkout::prepareGuestQuote()

Way 2: using a fieldset’s aspects

2.1. sales_convert_order (it is not used anywhere)

2.2. customer_account

See also: