SMTP stands for Simple Mail Transfer Protocol. It is an internet standard protocol that is used for sending email messages between servers or sending emails across the Internet. SMTP is used when email is delivered from an email client, such as Microsoft Outlook, Office 365, Apple Mail or Gmail to an email server or when email is delivered from one email server to another. When you receive an email in your inbox, most likely it is being sent from an SMTP server.
Click here to learn how to use Google SMTP server.
Click here learn about SMTP relays.
Click here to learn how to setup smtp in your email client.