In all likelihood they have hijacked another computer far away. Then they borrowed your email(like borrowing your name). That spoof only makes it look like you sent the email. In reality your computer and email account never touched it until it was returned to you.

In the snail mail world. This would be the same thing and mailing thousands of letter with another person's return address.

There is very little you can do to stop this. Someone who can read headers may be able to figure out that ISP the hijacked computer is actually related to allowing you to try and contact that ISP to intervene on that machine/IP address.

The only way to protect yourself here is to limit where you share your email address. Each time you put an email address on a forum or use it as a login you are at risk. Many create a second email to use when they must share their email address in the public. Would you put your phone number on the internet? Treat the email you use for your business of personal two way communications the same.

The spew will likely continue until the other machine hijacked is cleaned or disconnected from the internet.

Root cause someone found your email address laying around, hijacked another computer with tons of email addresses, stole all those email addresses, and is using the hijacked computer to spew many, many fraudulent emails. YOu are getting the emails that bounce back to your return address. The ones that get through look like leads to the recipient and they are calling or emailing you to get the business. Unless someone gets a ransom note? These are likely kids in a coffee shop having fun.