here are more an on DNS-Service configured to the box where the GWIA is
running. If one of them is not reachable the gwia tooks 27 seconds to
send a 220 response.

But my clients are wait only for 5 seconds and after this they cut the
connection :-\

Is there a way to prohibid GWIA does a reverse lookup? (Hint: Even the
reverse lookup is not successful the GWIA will established the
connection. So why the reverse lookup is doing?