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

When V8.10 sendmail starts up, it predefines the $={checkMIMEHeaders} class to contain five MIME headers: the Content-Disposition: header (Content-Disposition:); the Content-Id: header (Content-Id:); the Content-Transfer-Encoding: header (Content-Transfer-Encoding:); the Content-Type: header (Content-Type:); and the MIME-Version: header (MIME-Version:). You can add more headers with the C or F configuration file commands.

If any of these headers are found to be too long, they are truncated to the length specified by the MaxMimeHeaderLength option (MaxMimeHeaderLength). Note that this truncation is done carefully so as to maintain the appearance of an RFC2822-legal header.