How Can I Stop My Email from Being Spoofed?

It sounds like the issue partly stems from Bigpond being incorrectly configured. You could consider setting up measures to filter out any bounced emails that you didn't actually send to minimize your inbox clutter.

This seems related to Microsoft’s direct send feature. There are some updates on how they’re addressing these issues. You could look into the blogs about spoofing in Microsoft 365 - there might be a fix on the way for those who aren’t utilizing direct send.

If you've set up DMARC, SPF, and DKIM properly, there's not much else you can do beyond that. The bounces you’re seeing mean these spam emails aren’t reaching anyone, which is a bit of a silver lining, even though it’s frustrating to deal with. Unfortunately, you might just have to ride this out for a while until it calms down.

Bigpond is Telstra’s older email service. If you’re getting backscatter (which sounds like your situation), you might want to consider using the Advanced Spam Filter in O365, though it's not the most recommended option these days. It’s really tough to give a solid solution without checking your SPF/DMARC configurations and the actual email bounce responses you’re receiving.

Have you checked if it's truly spoofing or if your account’s been compromised? The high bounce rates with Bigpond could be due to a bunch of inactive mailboxes being closed recently. It might also mean someone could have extracted credentials and is using your domain incorrectly.

Don't forget to enable backscatter filter settings; they might help you manage these unwanted emails more effectively.