Beginning with V8.10,
sendmail is able to check headers with rule sets
(Section 25.5). When such a rule set is specified as
part of the H configuration command, and when that
header is found in a message, sendmail first
assigns values to special variables (such as
{hdrlen}, {currHeader}, and
{hdr_name}), then calls the appropriate rule set.
Should the calculated length of the header (as stored in
{hdrlen}) prove longer than the limit set at
compile time by MAXNAME (MAX...), the following
error will be logged and the too-long header will be truncated:
Warning: truncated header 'hdr' before check with 'ruleset' len=len max=MAXNAME
Here, hdr is the name of the header (the text to
the left of the colon). The ruleset is either
the number or name of the rule set that will be called to check this
header. The len is the length of the value (the
part of the header to the right of the colon) before it was
truncated. Generally, unless you redefined it, the maximum length is
set by MAXNAME, which defaults to 256 characters
(including the terminating zero-value character).
Note that this reports only that the value (the part of the header to
the right of the colon) is too long. If the name (the part of the
header to the left of the colon) is longer than 100 characters, it
will never be checked.
