Beginning with V8.10 sendmail, the MaxMimeHeaderLength option (MaxMimeHeaderLength) can be used to define the maximum length for the parameters that some MIME headers take. A parameter is separated from the main header name and value by a semicolon:

name: value  ;  parameter  ; parameter  ...

Before checking that parameter's length, sendmail looks to see if the header name is in the class $={checkMIMEFieldHeaders}. If it isn't, sendmail skips the parameter length check.

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

If any of these parameters are found to be too long, they are truncated to the limit imposed by the MaxMimeHeaderLength option (MaxMimeHeaderLength).