I sometimes make mistakes in sending email. (no offten) But I get emails from my ISP telling me that they could not deliver an email I never sent. What's happening here?

You need to check your computer for trojans/adware/spyware. Many turn your PC into a zombie and make it send e-mails without your knowledge.

Or someone out there has your address and is spoofing. In which case there's nothing you can really do to stop it.

I can give you a Gmail account if you want...PM with you addy...OR hotmail addy cause the EMAIL account you have is acting up