SMTP is short for Simple Mail Transfer Protocol. It is a set of communication directives that allows the forwarding of e-mail over the Internet. Most e-mail software is designed to use SMTP for communication when sending e-mail and will only work for outgoing messages.

SMTP provides a set of methods that simplify the communication of e-mail messages between servers. This is a kind of acronym that allows one server to divide different parts of a message into categories that the other server can understand.

Users only see the interface of an email, but when that message goes online, everything turns into strings of text. This text is separated by code words or numbers that define the purpose of each section. SMTP provides these codes, and email server software is designed to understand what they mean.

The other purpose of SMTP is to establish protocols between servers. In a typical SMTP transaction, a server will identify itself and specify the type of operation it is trying to perform. The other server will authorize the operation and the message will be sent. If the recipient address is incorrect or there is some other problem, the receiving server may respond with some kind of error message.

The strengths of SMTP are reliability and simplicity. Installing software that uses SMTP protocols is easy. Messages either reach a recipient or return an error message explaining why this is not possible.