I have setup bind on my ubuntu server and I have edited the file resolv.conf so that every client on the LAN gets the server ip as dns server.
But if I want to stay on the local network to reach my mailserver with my mobile phone through wifi it still goes to the outside.
How can I configure the server or bind or resolv.conf that all traffic for my own local domain stays inside and goes to my server?

1) Your wireless access point/router or whatever is running DHCP service for the wireless clients is configured to hand out your DNS server to clients? Or your DNS server is set up to use itself for DNS (in /etc/resolv.conf)?
2) Your DNS zone file contains an A record that EXACTLY MATCHES the mail server set up on your phone?

If the answer to these questions is "yes" then your phone isn't as smart as it claims to be. What are you using for DHCP? Is the server also on the wireless network? If not is it on the same (bridged) subnet? If you do an nslookup from other clients on the network does it use your server?