Milter is some sendmail's enhancement published under BSD license.
The Milter API is documented and this API can be implemented in another MTA.
Theoricaly, using Milter with other MTA (Qmail, Postfix, exim) would imply :
- writing some libmilter library implementing the functions in milter.h
- linking/recompiling against this counterpart library
Some other people would rather re-implement the protocol between
libmilter and sendmail.
This underlying protocol is currently not documented (except comments in
the sendmail source code).
It is furthermore internal hence subject to changes.
An attempt to reimplement Sendmail's Milter protocol in pure Perl
however does exist : PMilter
Its author has documented the "Milter protocol"