Hackers gain access to your email system by one of the following methods :
1. Brute force attack on weak passwords used by your users.
Typical passwords like password123, admin123, alice or <insert popular girlfriend’s name>, independent day dates, 012345678 should be avoided at all cost. This typically occurs at the hosting end.
Exploits are software bugs that belongs to existing OS, applications or libraries that upon triggering will allow the attacker to gain access to the system. This typically occurs at the hosting end.
3. Backdoor or malware in your notebook/machine/tabs.
A malware could do a key log on your system and inform the attacker of your passwords.
Hackers typically hack your email system to create a scenario of compromises. Some of these compromises can lead to further malware infection or targeted scams. Targeted scams happens when the attacker pretend to be your client/vendor and divert your accounting staffs to bank in the funds to the wrong account.
The best solution to prevent email hacking is to use AntispamEurope email security services.