People who are new to e-mail marketing often don't realise that not every sent e-mail reaches their recipients. Many reasons can compromise e-mail deliverability.

Let's try to identify the main reasons:
 

Your subscriber’s e-mail address is not correct. Your subscriber might have made a typing error when subscribing to your newsletter. Even a simple or silly error in typing the correct e-mail address invalids it. Remember that a few people forget their correct e-mail address, and give you a incorrect one.

The recipient's mailbox is full. It's not unusual that some people don't delete their old messages in their mailbox. Some mailboxes have a limited storage space. When the mailbox is full, all the new messages are rejected. The e-mail address no longer exists. It happens when old or unused mailboxes get deleted Network related problems. Sometimes they can occur when an ISP runs server maintenance or something goes wrong with the network. When e-mails don't reach their recipients and consequently they are returned to the sender, a so-called "bounce" is generated.

A "hard bounce" occurs when the error is permanent (i.e. when the e-mail address no longer exists). "Soft bounces" are related to temporary errors (i.e. when a recipient isn't reachable due to a network failure, the system tries to deliver the message again).
 

Another big problem in delivering your e-mail marketing campaigns is represented by spam filters. Even when you're not spamming, sometimes your e-mail can be considered as spam all the same. So, it gets deleted or moved into the spam folder of the recipient's e-mail reader.