An SMTP (Simple Mail Transfer Protocol) server is an application or computer that sends, receives, and relays email messages over the internet. It is a communication protocol used for sending and receiving email messages between email clients and servers. SMTP is a TCP/IP protocol that uses a reliable ordered data stream channel, typically a Transmission Control Protocol (TCP) connection, to communicate between the senders email client or server and the recipients email server. SMTP is used most commonly by email clients, including Outlook, Apple Mail, Gmail, and Yahoo Mail, to send messages from a sender to a recipient. SMTP can send and receive email, but email clients typically use a program with SMTP for sending email. An SMTP server is an outgoing mail server that handles the sending and relaying of outgoing email messages. It is an application that provides a service to other applications within a network, called clients. SMTP servers typically use TCP on port 25 or 587. Without an SMTP server, you cannot send your email to recipients.