Email Duping

I use Yahoo Mail Plus and have noticed several emails being sent to me from me. I don?t understand how this is possible since I change my password regularly. The only thing I can think of is people using my address as the return to address. Also, I?ve been receiving many messages for a failure delivery or timeout on messages which I?ve never sent but say they are from me. Is someone using my account or is this something else? I sit behind firewalls at home and at work and have us Trend Micros anti-virus software which is up-to-date. What?s going on?