Suggest strongly that you check if they traced you using your IP address or your e-mail address.
If it was using your e-mail address, then it is easy to "spoof" such an address - ie send an e-mail as if it has come from your address. Only the originating IP address will prove it wasn't you.
If they traced you using IP address, and if you have a static address, such as always on cable connection, then your ISP could be right and you did send the e-mail. If you have an open SMTP mail relay port (which is installed by default with certain software packages) then you could be acting as a relay agent for e-mails. Worse still, most Anti Virus progs won't pick up infected e-mails as they pass through the relay, so you are not infected but you are still relaying infected e-mails.