smtp – How do email blast services avoid their mail servers being blacklisteddue to email spamming?

Email blast services like Postmark, Mailchimp, SendInBlue and etc allows their users to send thousands of email to reach out mass contacts. Multiply that by the thousands of users they have, thats a LOT of emails hammering various SMTP servers.

I’m just wondering how these services are not been blacklisted by public/private Email servers world wide?

I would imagine the entire domain would have been blacklisted by now.

EDIT: I recently noticed that my an ‘’ email went into my spam folder.