Как-то так может помочь и отправляет электронное письмо только при изменении с ложного на истинное
#models.py
from django.contrib.auth.models import User
from django.db.models import signals
from django.db import models
from django.dispatch import receiver
from django.db.models.signals import pre_save
from django.conf import settings
from django.core.mail import send_mail
#signal used for is_active=False to is_active=True
@receiver(pre_save, sender=User, dispatch_uid='active')
def active(sender, instance, **kwargs):
if instance.is_active and User.objects.filter(pk=instance.pk, is_active=False).exists():
subject = 'Active account'
mesagge = '%s your account is now active' %(instance.username)
from_email = settings.EMAIL_HOST_USER
send_mail(subject, mesagge, from_email, [instance.email], fail_silently=False)