My iPhone sent SPAM mail without my knowledge

I have started to receiving autoreplies for some mail that was sent from my email address. A lot of autoreplies - more than 50 so far...

Some guy answered for this mail, and I saw then, that he were replying for email with .zip attachment and "sent from iphone" footer!

Any one knows how it's possible?

All happened today, and the only thing I've done today was reading received mails.

How it is possible that iPhone sent a lot of mails without my knowledge?

The phone is not jailbroken...