I have blogged before that the reason that I like MIMEDefang is that it gives the Postmaster a Perl interpreter (a programming language that is) and a library of functions that can be used to filter and manipulate incoming and outgoing email.
Of the functions available I believe that md_check_against_smtp_server() deserves special mention since it can be used to quickly implement a poor man’s milter-ahead or milter-sender. Of course milter-ahead implements many features (caching among others), but with some effort most (if not all) of the functionality can be implemented withing mimedefang-filter.
Then again, milter-ahead does not cost much (€90) even for small organizations, so the not invented here syndrome can be supressed.
Managing organized complexity 
..for those that don’t pay the MIMEDefang price already
Although milter-ahead, indeed, does not cost much.. smf-sav is a standalone milter, no perl (skills) required, and works like a charm :)
I’ve been thinking of reducing the performance penalty of Perl by building a MIMEDefang-like filter, but with another simpler language and implementation (tinyscheme comes to mind). However, I find it highly unlikely that my employer will agree to me implementing such a filter, since it will take a considerable amount of worktime (and maybe for many years; MIMEDefang has been around for ~10 years).
milter-ahead is not the only milter from SnertSoft that I am using, it is the only one that I have paid for. Plus I really like libsnert- very helpful when you want to hack a quick milter to test something.
Where I work we use smf-sav too and like Sotiris says, it works like a charm :)