In computer messaging, the Simple Mail Transfer Protocol or SMTP is an internet standard communication protocol that mail servers and other message transfer agents (MTAs) use to send and receive mail messages. User-level email clients typically use SMTP only for sending messages to a mail server for relaying. Receiving emails is typically done using the Internet Message Access Protocol (IMAP) protocol.
Port 25 is the default SMTP port for non-secure un-encrypted connections. Sometimes port 2525 or others for example can be used where Internet Service Providers (ISPs) block port 25. Secure connections are typically made over port 465 for SSL connections and port 587 for TLS connections.