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.