possible SMTP attack

Saturday, 31 July 2004

Hi,

I got this message in the logwatch sent to root:

Client quit before communicating:
    222.183.141.253 : 1 Time(s)

**Unmatched Entries**
   [222.183.141.253]: possible SMTP attack: command=AUTH, count=6: 1 Time(s)

What does it mean? How can I protect my server against SMTP attacks?

Thank you.

Olga