Why does Mail.app keep sent attachments open? Sending a pdf as an attachement leaves open the original file according to
lsof | grep my.pdf
How can i prevent this rather unpleasant behavior ?

OSX 10.6.8 - Mail Version 4.6 (1085)

UPDATE:
I filed a bug at apple and the guys at apple actually responded saying this is a know bug (duplicate of #4023875) which is still open.
I can't speak for why anything Apple has done, but three things come to mind for changing the open file status:


file a bug http://bugreporter.apple.com
quit mail
patch the Mail program to fix the bug


Unless you have some third party Mail plug in, it seems you've identified something that hopefully Mail eventually releases since you might have problems deleting a file that's still open.