Mechanic does not enforce a customer's marketing preferences on your behalf. It is your responsibility to respect their preference, as recorded in Shopify, and to give the customer an opportunity to unsubscribe if appropriate. If GDPR applies to you and your customers, this set of laws is also your responsibility to uphold, so far as it applies to sending email.
Furthermore, Mechanic does not allow bulk emailing. Only transactional email messages are permitted.
Read more: What kind of email can I send with Mechanic?