Over the past day we've detected someone from China using our server to send spam email. It's very likely that he's using a weak username/password to access our SMTP server, but the problem is that he appears to be using base64 encoding to prevent us from finding out which account he's using. Here's an example from the maillog:
May 5 05:52:15 195396-app3 smtp_auth: SMTP connect from (null)@184.108.40.206.broad.gz.jx.dynamic.163data.com.cn [220.127.116.11] May 5 05:52:15 195396-app3 smtp_auth: smtp_auth: SMTP user info : logged in from (null)@18.104.22.168.broad.gz.jx.dynamic.163data.com.cn [22.214.171.124]
Is there any way to detect which account it is that he's using?