Purpose
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Sometimes we could want to send a notification to the author too.
Like in the onboarding bars, where we send a sample invoice to the administrator.
He's the author of the message, but the goal is to send him an email to check the
full process when sending a sales order/invoice by email to a customer.
Specification
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For that purpose, if a key 'mail_notify_author' is set on the context when
sending the email, then notify the mail author.