There can be many reasons for the bounce of an email message, and the reason can be easily seen in the “Bounce message” or “Delivery Status Notification (DSN)” which will be sent by the mail system. Some of the most common reasons for the bounce back of an email message are:
>> Incorrect email address of the recipient.
>> The email of the recipient is no longer in use or expired.
>> Rejection of the email due to spam filter.
>> The limit of the mailbox of the recipient is full and it is not accepting anymore emails.