Hi Bob, thank you for the fast and very informative response. Indeed, I have bind9 installed and everything you wrote, is exactly like it is on my system.
I am sure, I misunderstood resolvconf, and your description explains, why domain resolution is still working, although the missing entry in resolvconf. Resolvconf is running for may years on my systems, but as I got some problems in the last two weeks to resolve my mailservers, I thought it would be a problem with resolvconf. However, might be another problem. Of course, I read all the manuals and documentations, but not all was clear to me (i.e. the bind9 thingi). So thank you for enlightening me. I think, this bug can safely be closed now. And please apologize for the noise. Best regards Hans