About null sender

[Feel Zhou]

Hello, my friend

This is Tom, I'm sending my greeting from China
I have some problem with my mail server, and I need your help
I have install postfix+amavisd+clamav+spamassassin in my mail server
My customer send mail via my mail server, It's working very well
For now, always have these log file
May  9 17:26:18 amavis[3938]: (03938-14) Passed CLEAN, [IP1] [IP2] <> -> <
someone at example.com>,
How can I make all this null sender mail as a spam email, and then it will
be blocked
Thanks for your help

TOM
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