gistfile1.py

for transaction in NewsletterTransaction.objects.filter(sent=None):
    ...

# to

while NewsletterTransaction.objects.filter(sent=None).count():
    for transaction in NewletterTransaction.objects.filter(sent=None).limit(10):
        ...

# taking 10 at a time and then doing a new query. this way you're 
# going to know if someone else is updating the db from anther place
# and can use this as a kill switch e.g. run into the db and set
# to some value will stop the process from completing the rest of the
# email send without having to kill the server process.