ALUG,
We get a few messages blocked for moderation because (I think) they are base64 encoded; that is, the Content-Transfer-Type header says "base64". (Note that this is distinct from Content-Type; a text/plain message can be base64 encoded.)
The pattern seems to be that all such messages are created using the Android mail client. A cursory Web search has turned up no solutions, but I did find an issue report:
http://code.google.com/p/android/issues/detail?id=2630
which I didn't read carefully enough to notice whether it includes a work-around or fix.
This message is merely for information.
Best, Richard