It is most unlikely your server is being used, unless you have compromised security. If that is the cae you have much more urgent processes to undertake than filtering sent mail (something Thunderbird can do on mail you send using Thunderbird)

Could it be your spammers are just using your email address in the from: part of their email. As I outlined earlier to you, backscatter is a real world response to spammers using legitimate email addresses in their spam. No using of anyone elses sever required to insert a false from email address.

Anything you send via Thunderbird will always be sent by you unless you allow someone to use your computer.

Just because email says it's 'From' you, does not mean the real sender sent via your email address and your server. If you receive any emails that say it's From you - You can check who is sending by looking at the 'View Source' info. In open email - 'More' > 'View source' Or use 'Ctrl+U' Or via 'Menu Bar' toolbar 'View' > 'Message source'

The 'Received from/by...' nearest the top will be last server to receive the email - which will be your server for your mailbox. The 'Received from' nearest the bottom - closest to actual message - is the first smtp server to get message and that will say who it's from initially, but it might not be a specific email address, it might tell you about the IP address.

The reason I ask is that a company called ProofPoint has begun intercepting my personal emails to companies and sending me bounce messages. I have tried contacting them and there is no response. Apparently, companies trust them to screen their email but ProofPoint simply claims that some senders are blacklisted, with no evidence. A nice way to make money, but probably fraudulent, and frustrating for me, because I'm actually not a spammer.

Yes, you are a spammer. You actually say you are here. https://support.mozilla.org/en-US/questions/1467081#answer-1679028