What is SMTP?

SMTP (simple mail transfer protocol) is an application layer protocol that is used to send emails from the sender’s mail server to the receiver’s mail server.

Steps

Suppose that person $A$ wants to send an email to person $B$. The steps involved in this email transfer are:

1. $A$ composes an email on their user-agent (e.g., Outlook) and adds the recipient’s email address.

2. $A$'s user-agent sends the email to $A$'s mail server.

3. The SMTP client process on $A$'s mail server opens a connection with the SMTP server process on $B$'s mail server by establishing a TCP connection on port 25 and conveying information (e.g., the sender’s email address and the receiver’s email address) to $B$'s mail server during handshaking.

4. The SMTP client process on $A$'s mail server sends the email to the SMTP server process running on $B$'s mail server. Upon reception, the email is placed in $B$'s mailbox.

5. $B$ can now pull the email from their mail server using a protocol like POP3 or IMAP.​