Are you trying to solve the problem of stable names for other services? You want to use the same name (smtp.fake) in sendmail config regardless of providers. Running a local nameserver for the .fake zone will do this. But then you need to change the .fake zone file on each connection. And change the forwarding line (or root zone file) as above to make the resolution of other names work.

Yes, thats what I want. But I want to use nsupdate to change the .fake zone. So it's changing the forwarding line thats the problem. That was the reason for the question.