def update(self, instance, validated_data):
        email_updated=False
        email = self.validated_data["email"]
        print(instance.email) #abc@email.com
        if email!=instance.email:
            if User.objects.filter(email=email).exists():
                raise serializers.ValidationError("email is not available")
            else:
                email_updated=True
        instance.__dict__.update(**validated_data)
        instance.save() # instance is saved.
        print(instance.email) #xyz@email.com
        if email_updated:
            task_send_activation_mail.delay(instance.id)#this one here
        print(instance.email) #xyz@email.com
        return instance

from `celery` import shared_task

@shared_task
def send_activation_mail(user_id):
    from project.models import User
    user = User.objects.get(pk=user_id)
    subject = 'Activate Your '+DOMAIN_SHORT_NAME+' Account'
    message = get_template('registration/account_activation_email.html').render({
        'domain_url': DOMAIN_URL,
        'domain': DOMAIN,
        'domain_short_name': DOMAIN_SHORT_NAME,
        'domain_full_name': DOMAIN_FULL_NAME,
        'domain_email': DOMAIN_EMAIL,
        'domain_support_email': DOMAIN_SUPPORT_EMAIL,
        'domain_support_url': DOMAIN_SUPPORT_URL,
        'mobile_support': MOBILE_SUPPORT,
        'user': user,
        'uid': urlsafe_base64_encode(force_bytes(user.pk)).decode(),
        'token': account_activation_token.make_token(user),
    })
    user.email_user(subject, DOMAIN_FULL_NAME +' ', html_message=message)
    return user.email #"abc@email.com" is printed as celery output.