SMTP DATA explanation

what it is:

tells the server that hey im ready to send the actual mail now. with headers and the body. needs the sender and recipient to be set beforehand.

interaction flow:

user: DATA

server: 354 Start mail input; end with <CRLF>.<CRLF>

• means send me everything upto a line with a single dot

user: sends the headers and the data and ends it with a single dot. example below:

Subject: Hello there
From: you@domain.com
To: them@example.com
Date: Thu, 22 May 2025 14:00:00 +0600
 
Hey! This is the body of the email.
It can span multiple lines.
.   ← a single dot on its own line ends it

server: final response 250 OK: queued as 12345

Key details

• Headers vs Body
  • The first section (before the blank line) are headers—Subject:, From:, To:, Date:, etc.
  • A single blank line signals the switch to the body.
• CRLF line endings
  • SMTP requires <CR><LF> (carriage return + line feed) at the end of every line.(telnet generates it automatically for you so no need to actually type the literal \r\n in. unless you’re scripting and doing it systematically)
  • CRLF line ending explanation
• End marker (“.”)
  • A line with just a . (and nothing else) ends the DATA.
  • If your body needs a line that starts with a dot, you “dot-stuff” by prefixing another dot. Server strips the extra on receipt. example:..This line actually starts with one dot
• Size limits
  • Many servers enforce a maximum message size (e.g. 10 MB). If you exceed it, you’ll get a 552 Message size exceeds fixed limit error.
• Transparency
  • Dot-stuffing ensures that the end-of-data marker can’t accidentally appear in your content.

Common pitfalls

• Forgetting the blank line between headers & body → server treats your first body line as a header and errors.
• Using just LF instead of CRLF → some servers reject it.
• Omitting the final dot or placing it incorrectly → server hangs waiting for more data.
• Not dot-stuffing leading dots → premature termination or mangled content.