Have you ever missed an important event, because you haven't got the invitation email, but an invitation email was evidently sent to you?
If so, you are probably using an email address at hotmail.com or outlook.com or in general Microsoft's mail services, also including addresses at live.com and other domains.
The reason for this is Microsoft's questionably spam protection. When an email arrives at their servers, the email is nearly always accepted, but then, if they think the email came from a server, which sends spam, the email is silently dropped. Neither the sender will ever get a notification, that the email didn't arrive, nor the recipient will be notified, that there was an email for her or him, respectively. The email is not even delivered to the recipients spam folder. The email is lost.
In terms of paper mail: its the same as the sender brings the paper mail to the post office, the sender pays for a stamp, the post officer accepts the mail, postmarks the mail, and as soon as the sender turns aways from the counter, the post officer throws the mail into the trash under the desk.
If you want to be reachable by email, you must not use Hotmail or any of Microsoft's mail services. You should use Gmail or any other mail service instead, where the sender gets an appropriate error message if an email is not accepted or delivered to the recipient. In case of an error message the sender can take action to fix the problem with the mail server, that might send spam, or the sender can try to contact you over another path, like snail mail or telephone.