possible SMTP attack

["Olga" <olga@xxxxxxxxxxxxxx>]

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