The feature is new in the Solaris Express 4/04 release.
MILTER, sendmail's new mail filter API, permits third-party programs to access mail messages as they are being processed in order to filter meta-information and content. This functionality, introduced in the Solaris 10 3/05 release, requires the following:
sendmail binary must be compiled with -DMILTER, which has been available since the Solaris 9 release.
The file /usr/lib/libmilter.so, which is in the Solaris 10 OS, must be available.
These files, /usr/include/libmilter/mfapi.h and /usr/include/libmilter/mfdef.h, must be available.
Both these files are included in the Solaris 10 OS.
Thus, with the Solaris 10 3/05 release, the user can build the filter and configure sendmail to use it.
For further information about sendmail, see System Administration Guide: Network Services.