The most likely reason is that your MX record is not configured or it still needs time to propagate over the internet. In your "Welcome Email", you have received the record, which you need to make at your DNS provider, so your incoming emails know how to locate the mail server of your domain. Usually DNS propagation takes up to 48-72 hours to go live, but we have seen it to be around 4-6 for most TLDs. This also depends on your DNS provider as well.