SMTP stands for Simple Mail Transfer Protocol, an application layer protocol. The client who wants to send the mail opens a TCP connection to the SMTP server and sends the mail via this connection. The SMTP server is always on listening mode. As soon as it listens for a TCP connection from any client, the SMTP process initiates a connection on that port. After successfully establishing the TCP connection the client process sends the mail. The most common port for SMTP are 25,25 for sending out without encryption. 465 and 587 are used with SSL/TLS/STARTTLS.
To send and receive messages you need either an email client or a web-based mail service that uses the SMTP to send emails and POP3 or IMAP (preferably both) for receiving them. Most email providers use all the standards needed. You should also inquire if they offer secure email connections. If you want free email hosting for your site and you’d like to send emails to your users, check that your hosting provider gives free SMTP access.