Simple Mail Transfer Protocol (SMTP) is a protocol for sending mail on the Internet. 

SMTP is a simple text protocol. Protocol running on TCP port 25 to send an email to the address. Need to apply MX (Mail eXchange) of the DNS. 

Mail transfer agent is present over 50 programs that can be used by the SMTP Sendmail program is the first program to use SMTP to another, including Postfix, qmail, Microsoft Exchange.

SMTP is Outgoing Mail Server

E-mail client must use the name or IP address of the SMTP server as part of the set. Server will send messages on behalf of users. This setting allows multiple policies and network design. End users connect to the Internet can use the email services do not need the same connectivity services (ISPs), network topology or the location of customers within the network or outside the network with no limit. factors for e-mail sent or delivered. SMTP servers usually use the modern client certificate (check) instead of a customer location (IP address) to verify that it is eligible to relay mail.

Server administrators choose whether clients use TCP port 25 (SMTP) or port 587 (Submission), as formalized in RFC 4409, for relaying outbound mail to a mail server. The specifications and many servers support both. Although some servers support port 465 for legacy secure SMTP in violation of the specifications, it is preferable to use standard ports and standard ESMTP commands according to RFC 3207 if a secure session needs to be used between the client and the server. Some servers are set up to reject all relaying on port 25, but valid users authenticating on port 587 are allowed to relay mail to any valid address. A server that relays all e-mail for all destinations for all clients connecting to port 25 is known as an open relay and is now generally considered a bad practice worthy of blacklisting.