Logged In: YES 
user_id=45365

That confirms my suspicion that this is a third-party error: filedescriptors 2 and 3 are readwrite on BSD, write-only on Linux.

A final test to confirm this: on BSD, add code like this just before calling popen():
os.close(1)
os.close(2)
os.open("/dev/null", "w")
os.open("/dev/null", "w")

If this makes the hang go away then the real problem is that whatever program executes your .forward (qmail?) erroneously opens a read-write pipe for your stdout and stderr. You can't do this with pipe() AFAIK, but maybe with socketpair() or something.