Authentication attempt via smtp

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From: tim (tim_at_discussions.microsoft.com)
Date: 03/02/05 

Date: Wed, 2 Mar 2005 07:05:01 -0800

It appears that some hacker is trying to login to my mail server via the smtp
service/port.

My logs report "authetication attempt from xxx.xxx.xxx.xx as \web failed:
logon failure unknown user name or bad passsword." The ip address is
registered to some phony company in china, so contacting the ISP is useless.
My firewall is setup to only allow incoming and outgoing mail via port 25.
When i turn off my internet mail service, that stops the hacker for a few
days and then it starts a cycle again. Any suggestions avilable???

thanks..