This entry expands on the subject of line endings in email (and news) messages, which I introduced in my previous entry on SMIME. In my (brief) experience working with mail and news messages, there are three different contexts involving line endings:
- transmission over the wire by SMTP (or NNTP)
- MIME canonicalization
- local handling of messages (storage, mail applications, etc.)
Transmission by SMTP RFC2822 specifies CR+LF line endings for on-the-wire transmission by SMTP.
MIME canonicalization RFC2049 specifies that CR+LFs are used in the canonicalization of MIME body parts BEFORE applying transfer encoding. It further states that
The output of the encoders may have to pass through one or more additional steps prior to being transmitted as a message. As such, the output of the encoder may not be conformant with the formats specified by RFC 822. In particular, once again it may be appropriate for the converter's output to be expressed using local newline conventions rather than using the standard RFC 822 CRLF delimiters.
In this context then, the appropriate linefeeds for an email message depend on the steps to perform after MIME canonicalization, and the local line endings convention.
Local handling It is apparent from the previous excerpt from RFC2049 that local handling of mail messages has to follow the local operating system's line endings convention. The most typical examples of this context are storing messages (eg in mailbox files or Maildirs), and piping/passing messages to mail handling programs like qmail-inject or Openssl's smime command. Googling on this context reveals the complexity often surrounding the question of proper line endings in email and news messages, and produces a few interesting links such as Life With Qmail: G.11. Carriage Return/Linefeed (CRLF) line breaks don't work, PHP bug #15841, a thread on the ietf- 822 list, and a thread on the Usenet Article Standard Update mailing list.
An illustrious victim of the line endings complexity I had to struggle with these past few days is Openssl's smime command, which has the nasty habit of outputting only the pkcs7 payload with CR+LF line endings when clear signing a message. The Python email package also seems to have a few problems with line endings, cf. bug #975330 submitted by Anders Hammarquist, who gave the very interesting Real-world email handling in python presentation at EuroPython 2004.