The commonest way this happens is when you end up in the address book of somebody who uses a Microsoft email program. Their system gets penetrated by malware, the address database gets passed on to a spammer or adopted as an alias by a viral remailer, and suddenly everybody the dimwit Microsoft user knows is getting spam or virus emails purporting to be from you. Chances are many of them will be too thick to understand any explanation of why it isn't your fault.

I have a few spare email accounts whose addresses I don't give out to anyone using MS software of any kind, so I'll always have a bolthole when the spam load makes all the others inoperational.