Handle invalid email if mandatory verification is enabled at a later time

[giohappy]

If ACCOUNT_EMAIL_VERIFICATION is set to mandatory at a later time, and we have users with invalid emails, next time they try to login will get a 500 error from Nginx, caused by an abrupt termination of the uwsgi connection due to an unhandled SMTPRecipientsRefused exception thrown by smtplib.

The /account/login POST request is handled by Django and the allauth lib, so at the moment we don't have a point where this can be handled gracefully.

A solution could be overloading the allauth.account.DefaultAccountAdapter.pre_login method inside our LocalAccountAdapter adapter.

In case the setting is set to mandatory this method will verify that a valid email is configured for the user. In case it isn't we can redirect to a page with a message inviting the user to contact the administrator for resetting their email, like we do for other similar messages like admin_approval_sent.html or email_confirm.html.
Several methods exist to verify emails in Python. One of the most common libraries is email-validator, but even something based on regex could be enough.

Moreover, this control should be skipped for administrators. At least for the default administrator.

Internal Server Error: /account/login/
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/site-packages/django/core/handlers/exception.py", line 47, in inner
    response = get_response(request)
  File "/usr/local/lib/python3.10/site-packages/django/core/handlers/base.py", line 181, in _get_response
    response = wrapped_callback(request, *callback_args, **callback_kwargs)
  File "/usr/local/lib/python3.10/site-packages/django/views/generic/base.py", line 70, in view
    return self.dispatch(request, *args, **kwargs)
  File "/usr/local/lib/python3.10/site-packages/django/utils/decorators.py", line 43, in _wrapper
    return bound_method(*args, **kwargs)
  File "/usr/local/lib/python3.10/site-packages/django/views/decorators/debug.py", line 89, in sensitive_post_parameters_wrapper
    return view(request, *args, **kwargs)
  File "/usr/local/lib/python3.10/site-packages/allauth/account/views.py", line 149, in dispatch
    return super(LoginView, self).dispatch(request, *args, **kwargs)
  File "/usr/local/lib/python3.10/site-packages/allauth/account/views.py", line 77, in dispatch
    response = super(RedirectAuthenticatedUserMixin, self).dispatch(
  File "/usr/local/lib/python3.10/site-packages/django/views/generic/base.py", line 98, in dispatch
    return handler(request, *args, **kwargs)
  File "/usr/local/lib/python3.10/site-packages/allauth/account/views.py", line 105, in post
    response = self.form_valid(form)
  File "/usr/local/lib/python3.10/site-packages/allauth/account/views.py", line 162, in form_valid
    return form.login(self.request, redirect_url=success_url)
  File "/usr/local/lib/python3.10/site-packages/allauth/account/forms.py", line 196, in login
    ret = perform_login(
  File "/usr/local/lib/python3.10/site-packages/allauth/account/utils.py", line 168, in perform_login
    response = adapter.pre_login(request, user, **hook_kwargs)
  File "/usr/local/lib/python3.10/site-packages/allauth/account/adapter.py", line 412, in pre_login
    send_email_confirmation(request, user, signup=signup, email=email)
  File "/usr/local/lib/python3.10/site-packages/allauth/account/utils.py", line 319, in send_email_confirmation
    email_address.send_confirmation(request, signup=signup)
  File "/usr/local/lib/python3.10/site-packages/allauth/account/models.py", line 59, in send_confirmation
    confirmation.send(request, signup=signup)
  File "/usr/local/lib/python3.10/site-packages/allauth/account/models.py", line 166, in send
    get_adapter(request).send_confirmation_mail(request, self, signup)
  File "/usr/local/lib/python3.10/site-packages/allauth/account/adapter.py", line 549, in send_confirmation_mail
    self.send_mail(email_template, emailconfirmation.email_address.email, ctx)
  File "/usr/src/geonode/geonode/people/adapters.py", line 139, in send_mail
    msg.send()
  File "/usr/local/lib/python3.10/site-packages/django/core/mail/message.py", line 284, in send
    return self.get_connection(fail_silently).send_messages([self])
  File "/usr/local/lib/python3.10/site-packages/django/core/mail/backends/smtp.py", line 109, in send_messages
    sent = self._send(message)
  File "/usr/local/lib/python3.10/site-packages/django/core/mail/backends/smtp.py", line 125, in _send
    self.connection.sendmail(from_email, recipients, message.as_bytes(linesep='\r\n'))
  File "/usr/local/lib/python3.10/smtplib.py", line 901, in sendmail
    raise SMTPRecipientsRefused(senderrs)
smtplib.SMTPRecipientsRefused: {'ad@m.in': (550, b'5.1.1 aDwdpOcoUgdVgaDwepjjm4 m.in dominio non valido / invalid destination domain')}