I get the error below when I try to send the invitation email to new users. 

Traceback (most recent call last):
  File "F:\Odoo 15\server\odoo\addons\base\models\ir_mail_server.py", line 584, in send_email
    smtp.send_message(message, smtp_from, smtp_to_list)
  File "F:\Odoo 15\python\lib\smtplib.py", line 967, in send_message
    rcpt_options)
  File "F:\Odoo 15\python\lib\smtplib.py", line 881, in sendmail
    raise SMTPRecipientsRefused(senderrs)
smtplib.SMTPRecipientsRefused: {'email@something.tld': (550, b'Verification failed for \nNo Such User Here"\nSender verify failed')}

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "F:\Odoo 15\server\odoo\addons\base\models\ir_http.py", line 237, in _dispatch
    result = request.dispatch()
  File "F:\Odoo 15\server\odoo\http.py", line 687, in dispatch
    result = self._call_function(**self.params)
  File "F:\Odoo 15\server\odoo\http.py", line 359, in _call_function
    return checked_call(self.db, *args, **kwargs)
  File "F:\Odoo 15\server\odoo\service\model.py", line 94, in wrapper
    return f(dbname, *args, **kwargs)
  File "F:\Odoo 15\server\odoo\http.py", line 348, in checked_call
    result = self.endpoint(*a, **kw)
  File "F:\Odoo 15\server\odoo\http.py", line 916, in __call__
    return self.method(*args, **kw)
  File "F:\Odoo 15\server\odoo\http.py", line 535, in response_wrap
    response = f(*args, **kw)
  File "F:\Odoo 15\server\odoo\addons\web\controllers\main.py", line 1339, in call_button
    action = self._call_kw(model, method, args, kwargs)
  File "F:\Odoo 15\server\odoo\addons\web\controllers\main.py", line 1327, in _call_kw
    return call_kw(request.env[model], method, args, kwargs)
  File "F:\Odoo 15\server\odoo\api.py", line 464, in call_kw
    result = _call_kw_multi(method, model, args, kwargs)
  File "F:\Odoo 15\server\odoo\api.py", line 451, in _call_kw_multi
    result = method(recs, *args, **kwargs)
  File "F:\Odoo 15\server\odoo\addons\auth_signup\models\res_users.py", line 202, in action_reset_password
    template.send_mail(user.id, force_send=force_send, raise_exception=True, email_values=email_values)
  File "F:\Odoo 15\server\odoo\addons\mail\models\mail_template.py", line 307, in send_mail
    mail.send(raise_exception=raise_exception)
  File "F:\Odoo 15\server\odoo\addons\mail\models\mail_mail.py", line 308, in send
    smtp_session=smtp_session)
  File "F:\Odoo 15\server\odoo\addons\mail\models\mail_mail.py", line 414, in _send
    msg, mail_server_id=mail.mail_server_id.id, smtp_session=smtp_session)
  File "F:\Odoo 15\server\odoo\addons\base\models\ir_mail_server.py", line 595, in send_email
    raise MailDeliveryException(_("Mail Delivery Failed"), msg)
Exception

The above exception was the direct cause of the following exception:

Traceback (most recent call last):
  File "F:\Odoo 15\server\odoo\http.py", line 643, in _handle_exception
    return super(JsonRequest, self)._handle_exception(exception)
  File "F:\Odoo 15\server\odoo\http.py", line 301, in _handle_exception
    raise exception.with_traceback(None) from new_cause
odoo.addons.base.models.ir_mail_server.MailDeliveryException: ('Mail Delivery Failed', 'Mail delivery failed via SMTP server \'None\'.\nSMTPRecipientsRefused: {\'email@something.tld\': (550, b\'Verification failed for \\nNo Such User Here"\\nSender verify failed\')}')

The email configuration works when tested.