Understanding basic SMTP commands and responses

There are only a few SMTP commands commonly used, and more importantly only a few responses that matter.

SMTP responses
Recipient systems will respond to each SMTP command with a numeric value and optional text. Any value in the 200-299 range is considered to be an "OK" acknowledgement.

HELO / EHLO
This is how the sending system opens an SMTP conversation with a recipient system that acknowledges a TCP 25 communication attempt. HELO is the original SMTP specification, EHLO is an ESMTP command. Parameters after the command are optional, although it should be noted that some recipient systems may attempt to match that against the domain name indicated by performing a reverse-DNS lookup of the sending IP address.

MAIL FROM:
This command displays the reply address of the sender.

RCPT TO:
This command displays the recipient address. Only one address is allowed per command, so messages with multiple recipients will show each separately.

DATA
This command signifies the start of the actual message. That includes what appears in the TO, CC, BCC, and Subject lines of the message. It includes the message body and attachments. None of that information is displayed in the SMTP log. ESMTP sending systems may declare the length of the message.

QUIT
This command requests a termination to the SMTP session.