If you ever add .o files to conf_prog_SRCADD (described earlier), and if those .o files need to be generated from .c files, you will need to list those corresponding .c files here:

define(`conf_sendmail_SRCADD', `myfilter.c')

Here, we add a source file to the source file list for the sendmail program only. To add source files to all programs, eliminate the _prog_ and use confSRCADD instead.

It is very unlikely that you will ever have to change this value from the one that is predefined for you in your devtools/OS file.