there's a few things that happen.
1. the site can only send like 300 emails per hour. Sometimes that limit is met very early within the hour causing notification emails to be discarded for the remainder of the hour. This happens about 3 days out of the week, about twice per day.
2. many email providers simply filter them out before they even reach your inbox. Yahoo is great at this. If you're using yahoo mail you won't even get your registration email. It won't be in your spam. It doesn't bounce back. It's just gone. Fuckin yahoo.
3. inbox spam folder. modern mail clients like gmail and outlook are watching you. When they see you getting the same emails day in and day out and they're never opened or clicked, your mail client learns and starts to filter these messages out. Sometimes your mail provider is nice enough to put them in a spam folder, other times, see #2