how am I getting email listed as from me in the sender box?

I am getting junk email from I don't know where saying it was sent by me.

What are exactly are you getting? Please be specific. Can you post an example?

The from box uses my email address. How is this possible?

Anyone can use your email address as From: address.

How is that possible? Every time I send an email, the from field is already filled in. I cannot change the from box to any email address I want to my knowledge. Thanks