Hello, hacking or entering another's computer system without authority is, indeed, a crime. You can look here to read the laws GA has in place regarding computer crimes such as hacking, computer trespass or theft and unauthorized access. Based on what you indicate it appears this may have happened given your explanation. In addition, as customer lists are normally private and proprietary information they are often considered trade secrets and, any unauthorized use or access to such information may also allow you to claim civil damages for such actions.
What evidence will the court consider proof? Is the receipt of a handwritten postcard three days later, by 4 persons who agreed to act as decoys, sufficient? Or is there enough room for doubt that 4 postcards were mailed to 4 of 5 million available addresses by pure coincidence?
Evidentiary proof, for a criminal case, must meet the "beyond a reasonable doubt" standard. To do that, it is likely more than just postcards may be needed. However, computer forensic work may be able to trace or detect any intentional intervention or taking of data back to the PC that actually took it. Paying for such work, however, can be costly. For a civil case (where the standard of proof is normally a preponderance of the evidence, a lower standard) that would mean that the person filing the case, the plaintiff, would need to pay for it. For a criminal case that would mean the government would want to pay for it enough to go ahead and do it (it does not have to) and may decide to use other evidence such as the post-cards you indicate as it may build a case in other ways as well. The numbers do indicate fairly unlikely that such a thing would occur, but a good defense attorney can easily argue such things prove nothing. There are no gaurantees with courts.
If, however, there are witnesses or persons who can believably testify that what you believe is happening actually is, then that can also help provide proof.
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