1.The email is dispatched, but for some unknown reason, it never makes it to the recipient's email address. The sender gets an automatic response that the email message did not get through. This response is typically called a bounced message.
2.Either the address no longer exists or you have mistyped or misspelled the address.
People change jobs and thus their work email is not longer valid.
People change their email address without notifying their subscription carriers.
Your email address lists are not being updated and cleaned, reflecting these changes.
3.Bad Internet connection,Website temporarily down and Server problems.
As for me, i used the Email Management Software of iKode Company to solve this problem.