One common reason for this is that notifications occasionally end up in a spam or junk folder. Checking these folders in your inbox and marking any notifications as not spam should resolve this.
Another common reason is if you are using Microsoft Outlook Enterprise as your mail server. Occasionally the email notifications are sent to quarantine and have to be manually released.
If you are experiencing this issue and you are not seeing notifications in your spam folder or using Microsoft Outlook Enterprise, please contact our support team.
It is also possible to connect a custom email domain, for more information please see this article: How do I connect a custom email domain?