Step 1: Understand the question.
We need the protocol that sends mail in an email system.
Step 2: Meet SMTP.
SMTP stands for Simple Mail Transfer Protocol. Its job is to push email from the sender's server toward the receiver's server.
Step 3: How sending works.
You write a message and press send. SMTP carries that message across the network to the recipient's mail server.
Step 4: Compare with receiving.
To pull mail down for reading, the receiver uses POP3 or IMAP. Those are for fetching, not sending.
Step 5: Reject HTTP and FTP.
HTTP is for web pages and FTP is for file transfer. Neither is the standard mail sending protocol.
Step 6: Conclude.
So the protocol that sends mail is SMTP, which is option 2.
\[ \boxed{\text{SMTP}} \]