When a header is checked using the $>+ in an H configuration command (Section 25.5.1), the unaltered value of the header is stored in the ${currHeader} macro and the length of that header's unaltered value is stored in this ${hdrlen} macro. Note that the value stored in ${currHeader} will be truncated to MAXNAME (MAX...) characters, the default for which is 256. If the header's value was longer than MAXNAME characters, the number of characters stored in ${currHeader} will differ from the value stored in ${hdrlen}. For an illustration of one way to use this macro, see Section 25.5.1.1.

${hdrlen} is transient. If it is defined in the configuration file or in the command line, that definition can be ignored by sendmail. Note that a $& prefix is necessary when you reference this macro in rules (that is, use $&{hdrlen}, not ${hdrlen}).