The ability to provide E-mail services to clients includes two critical functions: SMTP and POP3. Together, they provide the means for clients to send outgoing mail and check for new incoming mail, respectively.
SMTP service is the side of e-mail that allows clients to send outgoing e-mail messages to any valid e-mail address. The SMTP server performs two basic but important functions. First, it verifies that anyone attempting to send outgoing e-mail through the SMTP server has the right to do so. Secondly, it sends the outgoing mail and if undeliverable, sends the message back to the sender.
Most e-mail systems that send mail over the Internet use SMTP to send messages from one server to another; the messages can then be retrieved with an e-mail client using either POP or IMAP. In addition, SMTP is generally used to send messages from a mail client to a mail server. This is why you need to specify both the POP or IMAP server and the SMTP server when you configure your e-mail application.
You need to configure your e-mail client so that it knows what SMTP server to use for sending outgoing e-mail messages. In order to send mail through the proper SMTP server, configure your e-mail client to access the SMTP Server: yourdomain.com or IP address.