[EXIM] Froms & tos

How can a spammers mail end up in my box with his own address in the To:
field. IOW, I got a spam today like this:

From:       joeblow@???
To:         Joeblow@???
Subject:    Trillions and zillions to be made before 6:00 PM

How does this happen and why doesn't exim reject it?

Thanks

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