So how deep in number of posts are you under a subthread? Every post
(except the first one) contains a References header. It is used to
trace the parent-child relationship between posts so they can be
threaded into a hierarchy (i.e., a tree structure). That lets you see
who replied to who and in what order. If you are in a subthread that has
hundreds of replies in the chain then the References header can get
rather huge. Plus the Message ID used to define each post in the chain
is variable in length because the clients or servers define those
strings and they contain variable lengthed domain domains and different
lengthed tokens on the left side of the at-sign.
The recommended maximum length for any line, including headers, is 998
bytes. This is due to some servers or hosts truncating headers that
exceed that length. Programs run in memory, memory is limited, so
buffers to hold data in a message cannot be infinitely long. See
http://tools.ietf.org/html/rfc5322, section 2.1.1.
Continuation of a header is denoted by a space character in column 1
(actually I believe it is indentation using any number of whitespace
Headername: headervalue ....
is one header over 2 lines. However, I do not know that continuation
permits a [header] line to exceed the 998 character limit. It appears
mostly a readability feature, not a means of exceeding a length
limitation. The program will have to still deal with the total value of
the header (headerName + ": " + headerValue). While the header gets
folded into multiple lines, it is still handled as one header line.
One example of a header that too often violates the 998-character limit
is the X-Face header. Users of that header do not even consider how long
is that header but simply let their NNTP client figure out the value for
that ego-stroking superfluous header. The length of the X-Face header
could easily exceed the length of that poster's message. While my
newsreader supports the X-Face header (although I wish it had an option
to ignore it), it does support user-defined scripts which I may use to
strip out this garbage header (but scripts take time to modify the
content of received articles).
Since you did not bother to give the MID (message ID) of the post to
which you are attempting to reply, no one will know how long are the
headers in that post which your reply would make longer. If you want
something less than vague answers regarding a vague situation, you need
which you want to reply but which results in your NNTP client generating
an excessively long header line.