[drv]

(Assuming you mean MailChimp)

I don't know anything about how MailChimp operates, but a quick search turns up this blog post about how to set up SPF records [1].

From there, you can get a list of which IPs MailChimp authorizes as a sender; following the SPF include directive, you can see they specify two IPv4 ranges, both of which are in class C space, so it seems unlikely that MailChimp has their own class A for SMTP senders.

[1]: https://blog.mailchimp.com/senderid-authentication-for-your-...