Stands for "Simple Mail Transfer Protocol."
SMTP is the protocol used to send email over the Internet. Email clients (like Microsoft Outlook or the macOS Mail app) use SMTP to connect to a mail server and send email messages. Mail servers also use SMTP to exchange messages from one mail server to another. It is not used to download email messages from a server; instead, the IMAP and POP3 protocols retrieve messages.
An email client first establishes an SMTP connection over TCP with the mail server. The client then pushes the message to the server, which checks the recipient's email address to see if they're on the same domain as the sender; if they are, it can deliver the message. Otherwise, it runs a DNS query to find the IP address of the recipient's mail server. The mail server then establishes another SMTP connection with the destination mail server to send the message.
When configuring your email client, you may be required to specify separate mail servers for incoming and outgoing mail. While some email services use the same server for sending and receiving mail (e.g., mail.example.com), other services use separate servers for the two tasks (e.g., imap.example.com and smtp.example.com).
NOTE: Modern email clients and mail servers use a modified version of SMTP called Extended SMTP, or ESMTP. This updated version includes support for email attachments and TLS security.