What is the email subject length limit?

Question

How many characters are allowed to be in the subject line of Internet email? I had a scan of The RFC for email but could not see specifically how long it was allowed to be.

Answer 1

RFC2322 states that the subject header "has no length restriction"

but to produce long headers but you need to split it across multiple lines, a process called "folding".

subject is defined as "unstructured" in RFC 5322

here's some quotes ([...] indicate stuff i omitted)

3.6.5. Informational Fields
  The informational fields are all optional.  The "Subject:" and
  "Comments:" fields are unstructured fields as defined in section
  2.2.1, [...]

2.2.1. Unstructured Header Field Bodies
  Some field bodies in this specification are defined simply as
  "unstructured" (which is specified in section 3.2.5 as any printable
  US-ASCII characters plus white space characters) with no further
  restrictions.  These are referred to as unstructured field bodies.
  Semantically, unstructured field bodies are simply to be treated as a
  single line of characters with no further processing (except for
  "folding" and "unfolding" as described in section 2.2.3).

2.2.3  [...]  An unfolded header field has no length restriction and
  therefore may be indeterminately long.