To test with exim4 is easy. To reproduce on Ubuntu Jaunty:

1. apt-get install exim4-daemon-heavy

2. echo "local_scan = /usr/lib/exim4/local_scan/libpyexim.so" >
/etc/exim4/conf.d/main/15_py-exim_plugin_path

3. cd /usr/lib/exim4/local_scan

4. Compile mylib, output will be "libpyexim.so":

gcc `python2.6-config --cflags` -c -fPIC mylib.c -o mylib.o 
gcc -Xlinker -export-dynamic -lpython2.6 -lnsl -lpthread -ldl -lutil -lm
-lz -shared -Wl,-soname,libpyexim.so -o libpyexim.so  mylib.o

5. Restart server:
/etc/init.d/exim4 restart

6. Send some mail:
cat mail.txt | sendmail -t

(contents of mail.txt):
Content-type: text/plain
To: your_username@localhost
From: foobar@example.com
Subject: test

Hello world.