Security Concerns – unauthorized users hitting login page.

Our US-based rowing club website has had some interesting login attempts (apparently) from China. Last week I deleted a number of subscribers who clearly had no interest in our club, including [ email redacted ]. Yesterday we received an email failure notice from that domain, presumably because of a disk space limitation at the user’s end. I have already upgraded to the most current WordPress maintenance level (3.3.2). Is there anything else I can do to ensure that the website isn’t being hacked? The email in question is shown below. Thanks!

From: [email protected]
Date: April 25, 2012 6:31:08 AM CDT
To: [ email redacted ]
Subject: ??????

This message is generated by COREMAIL email system.
I’m sorry to have to inform you that the message returned

???????????????????????????????????????????????????????????????https://postmaster.163.net/

??????????????????У?????????????????????????????????????????????????????

?????[ email redacted ]??????????????????? : ?????????

The message to [ email redacted ] is bounced because : Quota exceed the hard limit

Username: [ userid redacted ]
Password: [ password redacted ]
https://noankrowing.org/wp3v20/wp-login.php