When sendmail reads lines of text from the configuration file or from qf queue files, it calls an internal routine named fgetfolded( ). That routine is initially passed a buffer of size MAXLINE into which to fit the read line. If the line is longer than MAXLINE, the sendmail program dynamically increases the space required to hold the line by MEMCHUNKSIZE.

When collecting the headers of a mail message, sendmail also begins with a buffer sized to MAXLINE. If a header arrives that is larger than MAXLINE characters, sendmail will increase the size of its buffer by MEMCHUNKSIZE as many times as is necessary to fully contain that header's data (up to but not exceeding the value of the MaxHeadersLength option (MaxHeadersLength).

The default value assigned to MEMCHUNKSIZE is 1024 bytes. If you need to change that value (for example, to port to a new system's strange malloc(3) requirements or for performance reasons), you must edit sendmail/conf.h:

# define MEMCHUNKSIZE   1024             /* chunk size for memory allocation */
                        
          change this to your new value

There is no debugging command-line switch to display this size for a precompiled sendmail. If this size is of concern, you must either discuss it with your vendor or download and build open source sendmail.