When you have sorted out the reasons why occur, you can then start fixing them. The most common reason behind this issue is poor security. Most businesses fail to use two-step verification that is recommended for all email accounts. When two-step authentication is enabled on an account, the system will require a valid email address from the user in order to proceed with the sign-in process. This is because the hacker will try to guess the correct password and try to obtain access into the system. The two-step authentication prevents this from happening.