Most E-mail services will only restore e-mails 7 days or less.
The Homeland Security may have them, but will not admit it violated the congress to keep them.
that depends how long they stay on yahoo's servers
You can't, only last 7 days
You can't. You can only retrieve mail from no longer than 7 days ago. Restore lost or deleted mail: https://help.yahoo.com/kb/sln2552.html