Some of my customer cannot send mail from a webserver.

If mail and web are installed on the same box, mail is serviced by qmail.
Step 1. Check /var/log/maillog for errors referring to the sender
Step 2. Log into the user CP, go to Web Options page for the domain mail is sent from, and view Error Log for errors related to sending mail.
The most common error is that there’s no mail relay from this mail server to the web server IP.
If mail and web are installed on separate boxes, mail is serviced by SendMail, not qmail. Check that the file /etc/sendmail.cf has the directive DS[MAIL_SERVER_NAME], for example DSmail.example.com. If it doesn’t, add this directive manually. This file can also be located in /etc/mail/