After you sign up with MailChimp, they will usually do an email verification to confirm that you own the domain.
Next would be to setup the SPF record, and here is what you need to add into your existing SPF record:
include:servers.mcsv.net
Afterwhich, you will need to setup the DKIM record, and here is what you need to add:
k2._domainkey CNAME dkim2.mcsv.net
k3._domainkey CNAME dkim3.mcsv.net
Many a times MailChimp would prefer that you set your DMARC to none
_dmarc TXT v=DMARC1; p=none;
