Django clean method of model doesn't gets called when saving the data, while its working for other models

So I having this model where I am storing the information that my entity is sending .

class NewProvisionalEmployeeMail(models.Model):
    offer_id = models.AutoField(primary_key=True)
    email = models.EmailField(max_length=70, null=False, blank=False, unique=False)
    token = models.TextField(blank=False, null=False)
    offer_sent_by = models.CharField(max_length=50)
    position_assigned = models.CharField(max_length=50, null=False, blank=False, default="Developer")


    def __str__(self):
        return str(self.offer_sent_by) +" to " + str(self.email)


    def clean(self):
        if(NewProvisionalEmployeeMail.objects.filter(email=str(self.email)).exists()):
            NewProvisionalEmployeeMail.objects.filter(email=str(self.email)).delete()

What was expected : Before saving data to this model, I expect the clean method to be called automatically. I have tried this with my other models over there it was working but its not working over here. This is the part where I am saving the data.

def sendoffer(request):
    context = {}
    hostname = request.get_host() + "/dummyoffer"
    if request.method=='POST':
        email = request.POST.get('email')
        position = request.POST.get('opt')
        context['success_message'] = "Mail has been sent!!!"
        token = get_secret_key(email)
        msg=EmailMultiAlternatives('ATG',"Hello\n we are from atg.dev.party\n Click on this link to get offer","[email protected]",[email])
        message = "http://" + hostname + "/" + token
        msg.attach_alternative(message,"text/html")
        msg.send()
        newusermail = NewProvisionalEmployeeMail(email=email, token=token, offer_sent_by=request.user.username, position_assigned=position)
        newusermail.save()

    return render(request, 'mainapp/offer.html',context)

newusermail.save() is the part where the data is going to be saved and before that I expect clean method to be called which checks for email if it already exists in our database and if it does we delete it.

I know there are several ways to achieve this but my question if why the clean method is not getting called?

EDIT : When I am overriding the save method and explicitly calling the clean method its working. Isn't the clean method gets automatically called. Why do I need to call it explicitly.

def save(self, **kwargs):
        self.clean()
        return super(NewProvisionalEmployeeMail, self).save(**kwargs)

It was however working fine for my other models.

EDIT 2 : I know we can do it using the save method but my question is why is it then working for my other models. For example in one of my model Works_in its working totally fine.

class Works_in(models.Model):
    emp_name = models.ForeignKey(User, related_name='works_emp_name', to_field='username', on_delete=models.CASCADE)
    dept_name = models.ForeignKey(Department, blank=False, null=False, to_field='name',related_name='works_on_dept', on_delete=models.CASCADE)

    class Meta:
        unique_together = ["emp_name", "dept_name"]

    def __str__(self):
        return str(self.emp_name) + " " + str(self.dept_name)


    def clean(self):
        emp_list = list(Works_in.objects.filter(dept_name=str(self.dept_name)).values_list('emp_name'))
        count = Emp_position.objects.filter(emp_uname__in=emp_list, position_name="Manager")
        is_manager = Emp_position.objects.filter(emp_uname = self.emp_name, position_name='Manager')
        # first the employee position should be added and then only department
        if(len(Emp_position.objects.filter(emp_uname = self.emp_name)) == 0):
            raise ValidationError(f"{self.emp_name}'s Position hasn't been added yet")
        if len(count)>=1 and len(is_manager)>=1:
            raise ValidationError('Manager already assigned to this department')