Using home computer (yahoo email) - are my emails recorded on the hard drive (even after deleting history)?

I would like to know if everything I do on my home computer can be retrieved by someone with intermediate computer knowledge (including the actual content of my emails - or is retrieving the content of emails a specialist role?
Update: Thank you for responding to my question but I'm as confused as ever as some of you say yes and some of you say no. How do the police track back terrorists emails?
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