newsletter_fora_do_site.py

from django.core.mail import EmailMessage
from contato.models import Newsletter
from django.contrib.auth.models import User

emails = ['lopinho@gmail.com']

newsletter = set(Newsletter.objects.values_list('email', flat=True))
cadastrados = set(User.objects.filter(email__in=newsletter).values_list('email', flat=True))
fora_do_site = newsletter - cadastrados

emails = "\n".join(fora_do_site)
email = EmailMessage('Emails do newsletter', 'em anexo', 'sistema-nao-responder@brbid.com', emails)
email.attach('newsletter.txt', emails, 'text/plain')
email.send()