Steps to reproduce the issue

1. Your site must have 2 users with access to the control panel and work e-mail addresses,
2. Log in to the control panel under one of the users, go to Components -> Private Messages -> My settings, enable the Lock Inbox option, save and log out,
3. Log in to the control panel as a second user and try to send him a message in Private Messages,
4. Log back into the control panel under the first user and check the incoming messages.

Expected result

Message must not be delivered. We should get an error or message:

if ($config->get('locked', false))
{
	$this->setError(JText::_('COM_MESSAGES_ERR_SEND_FAILED'));

	return false;
}

COM_MESSAGES_ERR_SEND_FAILED="The user has locked their mailbox. Message failed."

Actual result

The message was delivered to the first user without any problems.

System information (as much as possible)

Joomla 3.9.18
PHP 7.3.2