I have a service, example-service.com.
I have customers customer-a.com, customer-b.com, etc.
We host a service for them on urls like customer-a.example-service.com
Users who are familiar with customer-a.com and trust it might be sent to customer-a.example-service.com and be worried that it is some sort of scam/fake site. (indeed, this is a common phishing method, making a domain like paypal.com.payment-service.com, users quickly see paypal.com in the url and trust it).
The users are sometimes sent there from a third party, not from customer-a.com.
We have a "What is Example Service?" thing in the footer with some explanation. How else could we reassure our users about the relationship between the two services?