Line length limit
The rule that each line of an email message may hold at most 998 characters plus the CRLF (RFC 5322), and should hold no more than 78.
What it means
RFC 5322 sets two limits on every line of the header and body: a hard maximum of 998 characters, not counting the CRLF line ending, and a recommended 78, so that messages display well in simple programs. SMTP's matching rule in RFC 5321 is 1,000 octets per text line including the CRLF. The hard limit exists because many implementations cannot handle longer lines.
HTML generators sometimes write a whole message on one line. A server relaying it then has to break the line somewhere: Postfix, for example, breaks lines longer than its smtp_line_length_limit of 998 by inserting a line break and a space, which can split a tag or a link and break any DKIM signature made over the original. Encoding the HTML part as quoted-printable or base64 keeps every line short.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- RFC 5322, section 2.1.1: Line Length Limitsrfc-editor.org
- RFC 5321, section 4.5.3.1.6: Text Linerfc-editor.org
- Postfix configuration parameters, postconf(5)postfix.org
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