RFC 5322, "Internet Message Format", October 2008Source of RFC: IETF - NON WORKING GROUP
Area Assignment: app
Errata ID: 5918
Publication Format(s) : TEXT
Reported By: Marcus Bointon
Date Reported: 2019-11-26
Section 2.2 says:
Header fields are lines beginning with a field name, followed by a colon (":"), followed by a field body, and terminated by CRLF. A field name MUST be composed of printable US-ASCII characters (i.e., characters that have values between 33 and 126, inclusive), except colon.
I'm reporting an omission rather than a correction. The description of field names in S2.2 does not describe any length limit, but it implicitly prohibits folding by not permitting WSP chars in the name. 3.6.8 defines an ABNF for field-name, but does not specify a length limit either.
As far as I can see this means that field names should be limited to 77 characters – the field name and a trailing : – after which the field body can start after FWS on the next line.
I suspect this will be open to some debate, so I'm not sure what to suggest as a correction beyond that. Nor do I know whether common clients & servers impose their own arbitrary limits on field names.