The Mail Transport Agent (MTA) is what moves mail accross the internet or network. Generally the MTA recieves mail from the wire and either posts it to a mail store or relays it to another MTA.

This is the basic concept of an MTA, there are many different configurations and options. There can be many MTA's on a single corporate network.

You can find all the SMTP commands in section 4.1 of RFC 5321 SMTP Protocol produced by the Internet Engineering Task Force that we affectionately refer to as The Force.

SMTP Commands:

HELO/EHLO	Announce yourself. Your server client name or ip should follow. For EHLO the server responds with a list of SMTP extentions it supports.
MAIL FROM	who the message is from, the sender
RCPT TO	who the message is intended for (the ReCiPienT)
DATA	Preceeds the actual message or payload. Terminate with a dot on a line by itself
NOOP	Check the servers response. The server will respond with 250 OK if it can respond.
HELP	get a list of commands the server supports. A typical response: 214 AUTH STARTTLS HELO EHLO MAIL RCPT DATA BDAT NOOP QUIT RSET HELP Note: VRFY and EXPN are not in there due to security.
VRFY/EXPN	Verify if a mail box or mail list exists on the server
RSET	Reset the connection (wipe out any data already sent)
QUIT	Terminate the connection

This is an example of a simple SMTP session using telnet. The lines from the perspecive parties have been color coded for illustration. Also the lines from the server begin with S: and the client with C: - In reality, the server's response always begins with a number.

* - The server probably ran the data from these commands through an ACL of some sort.

The client could send to multiple recipients by issuing a RCPT TO command for each one

To send multiple messages to the same host, the client restarts with the MAIL FROM command once the server responds with a message Id.