Fending off attacks?

[GXX]

I've been blocking IPs left and right, trying not to set a /8 on an entire A class. But the attacks on trying to get into the email server just aren't stopping:

2006-07-17 14:04:02 H=(xx.xx.xx.xx) [218.234.229.176] F=<yiukcvbtu@ms41.hinet.net> rejected RCPT <kvivi911@yahoo.com.tw>: authentication required
2006-07-17 14:04:02 H=(xx.xx.xx.xx) [211.109.212.27] F=<udfmn@ms59.hinet.net> rejected RCPT <min145959@pchome.com.tw>: authentication required
2006-07-17 14:04:02 H=(xx.xx.xx.xx) [201.210.24.146] F=<bagvsspezralc@ms2.hinet.net> rejected RCPT <k630k@yahoo.com.tw>: authentication required
2006-07-17 14:04:03 H=(xx.xx.xx.xx) [203.128.85.13] F=<vgizwctbmf@ms53.hinet.net> rejected RCPT <s8553046@yahoo.com.tw>: authentication required
2006-07-17 14:04:03 H=(xx.xx.xx.xx) [58.233.13.61] F=<tqptktkleffjt@gyuvetch.bg> rejected RCPT <n0926780207@yahoo.com.tw>: authentication required
2006-07-17 14:04:03 H=(xx.xx.xx.xx) [211.109.212.27] F=<udfmn@ms59.hinet.net> rejected RCPT <kun1976@pchome.com.tw>: authentication required
2006-07-17 14:04:04 H=(xx.xx.xx.xx) [201.210.24.146] F=<bagvsspezralc@ms2.hinet.net> rejected RCPT <k63100@yahoo.com.tw>: authentication required
2006-07-17 14:04:04 H=(xx.xx.xx.xx) [58.233.13.61] F=<tqptktkleffjt@gyuvetch.bg> rejected RCPT <shadow1975@pchome.com.tw>: authentication required

Anyone have any ideas on what can be done? The IPs are constantly changing.