if i start hosting company i want to give my customer smtp and Pop 3.. how can i give this? what is the logic?
Yeah, it's possible. You need to setup their email address individually, which should not take much of your time. You'll need to make sure they don't use it for illegal purposes.
You may use your domain's mail server for outgoing mail rather than your ISP's if you configure your email client correctly. One of the two choices below should work. POP Before SMTP : If your POP3 Login name is the same as the email address to the left of the '@' symbol you may use "POP Before SMTP" which opens a window for you to send out through our server. SMTP Authentication : If you are not able to use "POP Before SMTP" you may use SMTP Authentication by configuring your email client. Check the box for "My outgoing server (SMTP) requires authentication"