Accessing sent emails
There are a number of options depending on exactly what you are trying to do.

Do you want to view mass mailings sent through CiviMail? If so Robin's answer should help.

Or do you want to view emails that you have sent to an individual contact in CiviCRM? If so, you can view these in that contact's activity tab.

Or do you want to record emails that you have sent outside of CiviCRM against a contact in CiviCRM?  If so, you can find more details here: Configuring inbound email processing