At what point in the SMTP conversation can the SMTP client send message headers?

Question

Options

• ABetween RCPTTO and DATA
• BBetween HELO and MAIL FROM
• CBetween DATA and a period"." on a single line
• DBetween MAIL FROM and RCPTTO

• A
  6% (4)
• B
  2% (1)
• C
  91% (58)
• D
  2% (1)

Explanation

In SMTP, the DATA command signals the server that the client is about to transmit the full message (headers + body). The server responds with a 354 go-ahead, and the client then sends everything - From:, To:, Subject:, and all other headers, followed by the message body - until it sends a lone period (.) on its own line, which tells the server the message is complete. That terminating dot is the only valid endpoint for message content transmission.

Why the distractors are wrong:

• A (RCPT TO → DATA): At this point the envelope is still being built; the server is only accepting recipient addresses, not message content.
• B (HELO → MAIL FROM): This is the greeting/identification phase - no envelope or message data is allowed yet.
• D (MAIL FROM → RCPT TO): The client is still specifying the sender and recipients in the envelope; message content hasn't started.

Memory tip: Think of the SMTP flow as an envelope being sealed, then filled. HELO→MAIL FROM→RCPT TO = addressing the envelope. DATA..."." = everything inside the envelope - and headers are always inside.

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