The first thing we have to understand is that the sending of emails relies on two actors, the one who sends (sender) and the one who receives (receiver). There are different reasons why the sender can send the email and it says sent successfully, but the recipient does not receive it and we will explain why.
It is also important to understand the difference between a transaction and a normal email. The transaction is an email that occurs as a result of an action of the client in our center: make a reservation, sign up for a tournament or a match. Those emails are a response to the action initiated by the client and therefore is not considered spam, even if the client is on the unsubscribe list.
The customer has the option to unsubscribe from receiving emails so you will have to check if they are on this list or not first.
If you look at the sent status and see that it is sent correctly, it means that the mail has gone out but does not guarantee that the recipient will receive it.
This email that appears to be sent correctly may have been moved by the recipient's mailbox to the spam or spam folder or even the recipient's mail server might block it without taking it to the recipient's mailbox or to the spam folder.
You can ask the customer who has problems receiving the email, to add the sending address to their address book and mark it as safe to prevent it from reaching the spam folder.
Increasingly, with the new GDPR law, the recipient is protected from receiving emails that they do not want to receive and the algorithms used by mail servers to catalog emails are increasingly advanced.
The email address to be marked as safe is firstname.lastname@example.org