Some reasons why an email is not delivered can be:
- No internet connection: you're trying to send an email but you're simply not connected to the internet
- The receiver's inbox is full: the receiver has many emails in his/her inbox and cannot receive any more emails unless he/she empties the inbox.
- The email was rejected by a spam or virus filter: the email was analyzed by a spam or virus filter which determined that the email was potentially malicious. In this case, the email may end up together with other spam emails or the email is simply deleted
- The recipient's email address was not recognized: you sent an email to an email address that simply does not exist (anymore).