Depends, if it was logged alongside the message then if they've got the message they've got the account associated with the message and they've got the IP address all in the same record. That's how a lot of forums work. As such I'd expect the IP address to automatically form part of the information Yahoo disclosed to the claimant's lawyers. I'm pretty sure you're entitled to see this correspondance. Ask the claimaints lawyers for it if you';re getting no joy from Yahoo.
If they'd have to fish the imformaiton out of web access logs on a seprerate system it's quite possible that these have been deleted or archived in a such a way as to not be considered searchable and therefore not covered by data protection legislation.
If archived then it may be possible for your lawyer to supoena them. I don't know.
As an ISP they should be convered by the the
Data Retention Directive That means that the data almost certainly was logged at one time or another.
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