I develop software and sell licenses to a client in a relationship that is technically non-exclusive but so far has been exclusive in practice i.e. they are an important client to me.
My client (let's call them ABC solutions) now wants me to start interacting directly with their customer (XYZ solutions), but only through their own company email address (firstname.lastname@example.org), and not from my own email. I'm not sure to what extent my client has clarified the nature of our relationship to their customer, but when talking to their customer, they call me "our developer". (It is true that I have occasionally consulted for them on unrelated work.)
My wish is to do whatever I can to streamline integration of technologies from all three parties, and for this reason I am prepared to interact directly with their customer. But I feel that this arrangement of interacting in a role of "their developer" through their email (perhaps allowing their customer to assume I am an employee) may be unprofessional and could lead to some unexpected pressures on me later on. Any advice would be appreciated. Is it reasonable for my client to request that I interact with their customer through an email address they provide?
Edit: further information drawn out in the comments:
My client is in Luxemburg, their customer is in France, and I'm in South Africa.
My client licenses each instance of the software from me. They've never paid me per hour except on consulting requests unrelated to the product they are licensing from me. I started building this software long before I had any relationship with the client.