Whatsoever may be the reason, you can easily disable all emails sent automatically or manually in WordPress through wp_mail() function.
WordPress Disable Email plugin is the new WordPress plugin that automatically disables all emails by default but also provides you option to send disabled emails to specified email address.
Start by installing ‘WordPress Disable Email Plugin’ in your WordPress website.
Upon activation, visit your admin area dashboard Settings -> Disable Email page to enter the host name of your website as the plugin performs a check on the $_SERVER['HTTP_HOST'] variable.
You can also set an override email address (optional) to force all emails to that specific email address. You may leave this field empty to just disable email.