The single biggest child site issue that you can end up dealing with is why is the site email not being sent. You could use an SMTP plugin and then use SMTP creds to connect. But using a transactional email provider makes it easier in knowing that your child site email will be delivered correctly. The delivery of site email can be a pain to deal with on child sites. If a site is using WooCommerce or a form plugin like Gravity Forms, you want to ensure that form notifications and customer order notifications are being sent out to customers.
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