I recently looked in my sent folder in my emails and it had over 50 email addresses in it saying I send emails to those addresses, and I didnt even send them.
The email content was of a online store advertising based in china what I have never heard of.
How as this happen because it is my own personal email address and no one else
A spammer has got your address and is using it to send out emails to possibly thousands of addresses. Naturally, they don't use their own, as they don't want to get all the notifications of failed delivery.
All you can do is either change email addresses or wait until the spammer chooses another address. This happened with one of my email addresses and I got 5000 undeliverable emails a day for abouta week.
Knowing someones email address wouldn't enable you to send emails from that address. Also, this would not account for them being in your sent folder.unless you have a web based email possibly but if you're on a computer and use an email program like Windows Mail, Thunderbird, Outlook Express or MS Outlook, then you have a virus I'm afraid
Although, as squarebear says, "Anyone can spoof the from address in an email to make it appear as though it comes from anyone they wish", that would not result in them being in the Sent E-mails folder of the spoofed address.