When a domain is authenticated on your SendGrid account, you will need to use the Custom DKIM Selector option to configure your Domain Authentication on a second account. This guide explains how to use a custom DKIM selector for these purposes.
Overview
Every Domain Authentication is generated with the same DKIM selector values for your DKIM CNAME records. These default values are S1 and S2, resulting in DKIM CNAME records of s1._domainkey.YourDomain.comands2._domainkey.YourDomain.com.
Using a custom DKIM Selector allows you to change these values to any combination of one to three letters or numbers.
How to set a custom DKIM Selector
The custom DKIM Selector option is located under Advanced Settings when you are initially setting up your Domain Authentication. In this example, we have set the custom DKIM Selector to a1.
On the next screen, you will see your new CNAME records generated with a Custom DKIM Selector.
The DKIM CNAME records generated while using a Custom DKIM Selector are now a1._domainkey.YourDomain.com and a12._domainkey.YourDomain.com. You can now enter these values into your DNS provider without causing a conflict with another Domain Authentication on a separate account or Subuser.