How can you get IP addresses of logins to a Yahoo! Mail account? - Quora
Firstly, the good news: The information certainly exists.
But: In New Zealand, where I am, there are no circumstances where the holder of that information could legitimately give the information directly to you in these circumstances.
And, I suspect, it is the same in other countries, if not by law then by Yahoo! company policy.
In New Zealand the process of investigation would probably involve laying a complaint with the police and having the information being delivered to them. It is very unlikely that this information would then be given to you by the police.
If this is a civil matter, the best I can suggest is that you have your investigator start by contacting the folk in charge at Yahoo! and see how they can help. I guess there are some jurisdictions where courts will issue an order to Yahoo! to release the information.
With an order, you might be able to force Yahoo! to allow logins only from nominated IPs.
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