We presume you haven't actually been sending spam and that you have found out by emails being bounced to you that you never sent or angry people emailing you not to send spam.
There are a few causes:
1. You have some kind of virus or trojan - these tend to use your address book and if the people are strangers then it might be another reason. You should update your virus scanner and firewalls now if you haven't already just to be safe.
2. If you are running an older version of FormMail (less than version 1.91) on your site this can cause security problems and allow other people to send out emails from your site (if you look at the FAQs server stats you can see the occasional hit from people looking for a copy of FormMail). Upgrade the script or use one of the patches or use a different script.
3. People may simply be adding your name to the reply to box. This does make it look like you've sent the email at first glance but all the information in the headers proves your innocence. If you have found this out from mail bouncing then forward the email (as an attachment) to your email provider and they should be able to confirm it isn't you and you can then direct any complaints to them. If people complain get them to send the email to their email provider and they should be able to prove your innocence.
and that you have found out by emails being bounced to you that you never sent or angry people emailing you not to send spam.