SMTP stands for Simple Mail Transfer Protocol. When someone speaks of a SMTP server they are referring to an outgoing mail server. Most hosting companies do not offer access to SMTP servers as they can be difficult to maintain and defend and the want to cut corners with their customers.
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.