E-mail delivery rate
People who is new to e-mail marketing often don't imagine that not all their sent e-mails will reach their recipients. Many reasons can compromise e-mail deliverability.
Let's try to analyze what the main reasons are.
The e-mail address of your subscriber is not correct. Your subscriber could make typing errors when subscribing to your newsletter. Even an unrelevant error in typing the correct e-mail address makes it invalid.
Consider that a few people forgive their correct e-mail address, and gives to you a bad one.
The recipient's mailbox is full. It's not a rare fact that some people don't delete the 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 gets deleted
Network related problems. Sometimes they can occurr when a ISP runs server maintenance or something goes wrong along the network.
When e-mails don't reach their recipients and consequently they are returned to 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 reacheable due to a network failure, the system tries to deliver again the message).
Anti spam filters
Another big problem in delivering your e-mail marketing campaigns is represented by spam filters.
Even you're not spamming, sometimes your e-mail can be considered as apam anyway. So it can be deleted or moved into the spam folder of the recipient's e-mail reader.