hacking mail?

I get a lot of these everyday:

May 20 02:04:51 srv1 postfix/smtpd[9411]: NOQUEUE: reject: RCPT from unknown[218.88.34.12]: 550 5.1.1 <[email protected]>: Recipient address rejected: User unknown in local recipient table; from=<[email protected]> to=<[email protected]> proto=SMTP helo=<163data.com.cn>
May 20 02:04:51 srv1 postfix/smtpd[9411]: NOQUEUE: reject: RCPT from unknown[218.88.34.12]: 550 5.1.1 <[email protected]>: Recipient address rejected: User unknown in local recipient table; from=<[email protected]> to=<[email protected]> proto=SMTP helo=<163data.com.cn>
May 20 02:04:52 srv1 postfix/smtpd[9411]: NOQUEUE: reject: RCPT from unknown[218.88.34.12]: 550 5.1.1 <[email protected]>: Recipient address rejected: User unknown in local recipient table; from=<[email protected]> to=<[email protected]> proto=SMTP helo=<163data.com.cn>
May 20 02:04:53 srv1 postfix/smtpd[9411]: NOQUEUE: reject: RCPT from unknown[218.88.34.12]: 550 5.1.1 <[email protected]>: Recipient address rejected: User unknown in local recipient table; from=<[email protected]> to=<[email protected]> proto=SMTP helo=<163data.com.cn>
May 20 02:04:54 srv1 postfix/smtpd[9411]: NOQUEUE: reject: RCPT from unknown[218.88.34.12]: 550 5.1.1 <[email protected]>: Recipient address rejected: User unknown in local recipient table; from=<[email protected]> to=<[email protected]> proto=SMTP helo=<163data.com.cn>
May 20 02:04:55 srv1 postfix/smtpd[9411]: NOQUEUE: reject: RCPT from unknown[218.88.34.12]: 550 5.1.1 <[email protected]>: Recipient address rejected: User unknown in local recipient table; from=<[email protected]> to=<[email protected]> proto=SMTP helo=<163data.com.cn>


how do I stop these people from trying to hack in?
How can I stop them by ip?
I have fail2ban installed but it does not catch this.

Fordwrench