There are a couple of different ways to organize your custom tracking domains.
Based on a per-business basis
Use a single custom tracking domain for multiple sending accounts. This would make sense if your primary domain is old/warmed up. If you're running a lead agency, this is what we would recommend.
Based on the associated domain for the email account
If you had [email protected] and [email protected], you may use:
inst.first.com for all email addresses associated with first.com
inst.second.com for all email addresses associated with second.com.
This approach would make sense when all of your domains are new, or if you're experimenting with a new campaign that could potentially have a negative impact on your reputation. By creating separate groups, you can minimize the damage from a bad campaign.
Note: For Done-For-You or Pre-warmed domains, we will set up the CNAME record and enable the custom tracking domain automatically.