I have CentOS 5.3 with ISPConfig 3. Email hasn't recieved by recipient if I send them using Email client such as Outlook. But email go out without any error return. If I send them using webmail, it it reach the recipient properly. I'm so curious about this. Are they not using the same SMTP?

Does the mail client claim to have sent the message? If so it's likely a configuration issue on the client. If it does claim its sent check the logs and see why it wasn't delivered. Provided smtp is listening on an address accessible from outside and you're allowed to send messages through it or it's responsible for the end termination of the message it should work fine. You didn't mention what mail server you're running, how you're setup, what webmail you're using, if its sending locally or through port 25, etc... there are a lot of variables that are in the open and unknown.

please check your mail client configurations.

may be there are some option you have to check, such as usage of SSL/TLS, need authentication, smtp server or anything else, because if it's succeeded with webmail, it should succeeded when using mail client,

regards.