The registrar can’t really do anything, and the service they use to receive email (what you’d see in the DNS MX record) is often totally different to the service used to send marketing emails. You’d need to look at the Received headers of the email to figure out where it was sent from. For example, a lot of companies use Office 365 or G Suite for corporate emails, but something like Mailchimp or ConstantContact for marketing emails.