I tried to solve the problem and i did the following:
1-the connectivity exists
2-i tried the OWA and it works.
3-No external/internal sending/recieving
4-the user is using some development applications, i stopped the services related to those applications, but nothing works.
5- i instructed the user to telnet the server but no response on smtp.
6-he tries his account on another machines and it works probably.
Please help me as soon as possible.