Whitelabeling DNS records
First make sure your own domain is verified using the SPF, DKIM, and Tracking records on the Domain Verification Guide - Your own customers can then add the following to their own DNS.
SPF
Name: @
Type: TXT
Value: v=spf1 a mx include:yourresellermaindomain.com ~all
Tracking
Name: tracking
Type : CNAME
Value: your.custombrandingcname.com
Do not change DKIM entry.