Due to repeated use of StringIO as a way to "look ahead" into subparts 
while checking that multipart boundaries are unique, memory consumption 
during email.generator.Generator.flatten() can be up to 3 times the 
original message size.

I implemented a subclass of email.generator.Generator that works around 
this using email.message.Message.walk() to check message headers and 
string (final) payloads for the boundary without duplicating their 
contents into a StringIO.

It assumes that the boundary only ever might be duplicated in a single 
part's headers or in a single part's payload when that part's payload is 
a string.  IOW, it assumes that the boundary will not be duplicated by 
some combination of all the parts' and recursive subparts' headers and 
string payloads.

If this assumption is safe, then this implementation should work.  If 
this assumption is not safe, then perhaps a different boundary format 
can be used which will make this assumption safe?

You can find my implementation at http://gitorious.org/rpatterson-
imappipe/rpatterson-
imappipe/blobs/master/rpatterson/imappipe/generator.py