Beginning with V8.10 sendmail, the MaxMimeHeaderLength option (MaxMimeHeaderLength) can be used to define the maximum length for selected MIME headers that present text descriptions. Before making that check, sendmail looks to see if a particular header is in the class $={checkMIMETextHeaders}. If it isn't, sendmail skips this length check.

When V8.10 sendmail starts up, it predefines the $={checkMIMETextHeaders} class to contain the single MIME header Content-Description: header (Content-Description:). You can add more headers with the C or F configuration file commands.

If this header's value is found to be too long, it is truncated to the length specified by the MaxMimeHeaderLength option. Note that this is a blatant truncation, and no effort is made to keep the header legal because it contains only random text.

Note also that you should use $={checkMIMEHeaders} ($={checkMIMEHeaders}) for RFC-format specific headers.