What are CR and LF?
- CR = Carriage Return (ASCII 0x0D)
- LF = Line Feed (ASCII 0x0A)
- Think typewriter: CR moves the carriage back to the left margin, LF advances the paper one line down.
Why both?
Back in the day, different systems used different line endings—some used just CR, some just LF, some both. To be crystal-clear across all systems, SMTP mandates the pair \r\n (CRLF) as the only valid end-of-line marker.
Where it matters in SMTP
- Commands: Every SMTP command (
HELO,MAIL FROM, etc.) must end with CRLF. - Headers & Body: Within the DATA block, each header line and each body line also needs CRLF.
- End-of-data marker: That lonely
.must be followed by CRLF to tell the server “yep, that’s the end.”
What can go wrong if you skip it
- Using just
\n(LF) can make the server wait forever, thinking your line is still going.- Some servers will flat-out reject the message with a syntax error.
- Dot-stuffing only works correctly if the server sees the exact CRLF sequence before the
..
How to ensure you’re doing it right
- If you’re scripting in Bash or Python, explicitly write
\r\n(not just\n).- Telnet/NC clients usually handle it for you—just hit Enter, and they send CRLF. But if you generate the session programmatically, watch out.
Basically, CRLF is the universal “end-of-line” handshake that keeps old and new systems in sync :3