SMTP is a set of rules for the transmission of messages from their origin to the destination and is used to transmit email messages to mail servers. SMTP uses port 25 to send or transmit mail messages. Email servers need to have this port open to listen for incoming connections.

In the connection between the client and server, the client sends the first SYN message to the server to start the connection through port 25. The server accepts the connection by sending the SYN_ACK message.

After this exchange of messages, the server sends the client a message with identifier 220, indicating that the server is ready to carry out transactions so that it can proceed and send emails. Subsequently, the client identifies the server message through HELO, which is used to read the messages.

The following image shows the client and server's communication through the SMTP protocol:

In this diagram we can see the first phase of the communication where the client sends a SYN packet to the server using the port 25. If the connection is established, a series of confirmation packets SYN_ACK are exchanged between them. Finally,the server it returns the  STMP server: READY packet to the client indicating that server its ready to receive connections.