The server sends a greeting
S:	220 We are Marlin - Pleased to make your acquaintance
The client announces itself with host name. It is permissible to use an IP address IAW RFC 5321
C:	HELO colt.example.com
Server acknowledges
S:	250 Marlin.devnet.leistware.com Hello colt.example.com [1.2.3.4]
Client gives the senders address
C:	MAIL FROM: john@example.com
Server accepts the sender*
S:	250 OK
Client sends the RCPT TO command followed by the recipient address. This command is repeated for multiple recipients.
C:	RCPT TO: kennyl@example.com
Server accepts the recipient*
S:	250 Accepted
Client indicates the message is comming.
C:	DATA
Server responds with instructions
S:	354 Enter message, ending with "." on a line by itself
Client sends the message headers followed by the message text. Then as instructed by the server a single dot on a line by itself.
C:	From: john@example.com To: kennyl@example.com Subject: SMTP Session Reply-To: john@example.com Example.com: Custom headers no longer begin with an X- An SMTP session can be initiated with software like telnet between a client and an MTA. Most software takes the MTA's host name or ip address followed by the port as arguments. .
Server responds with the message ID - which completes the transaction.
S:	250 OK id=1n59wq-00068T-DY
Client sends the QUIT command to terminate the session OR The MAIL FROM command to begin another transaction.
C:	QUIT
Server signs off
S:	221 Marlin.devnet.leistware.com closing connection