Some subscribers do not receive my newsletters

Numerous subscribers have told me that they do not receive my newsletters. When I check their email addresses, which are correct, it does appear to me as received. They are people who are waiting for the information, so they update their emails daily. I also receive some mails back (delivery failured) from people who have not received the newsletter (access denied, for example) and nevertheless in the plugin appears as read. Why is this happenning?