sendmail connection timeout

[supportadl]

Hi all. From some days I have a problem on my public mail server. It is running Centos 5.3 and sendmail 8.13.8. When I contact sendmail on port 25 to send an email it is very slow to answer and sometimes the connection timed out. If I make a telnet on port 25, the connection is open instantly but I need to wait for a lot of senconds (15-20) to see the line regarding hostname and sendmail service. I test it also directly on the server with the network ip address 'telnet 192.168.1.10 25' and it is slow, if I make a telnet like 'telnet 127.0.0.1 25' instead it is instant.
I also test mx resolution and it is instant.
Attach you find my configuration file, can you see something wrong? Anyone can help resolving this issue? Thanks a lot.