Log into FastMail’s Web mail and click on the Mail menu at the top left.
Click on Settings, then Spam Protection.
Make sure that Custom is selected:

Make sure that Move, Discard, and Add score are all unchecked.
You can optionally click Show advanced preferences, go to the Backscatter section, and select With bounce spam: Do nothing to have FastMail ignore messages where it thinks a spammer impersonated you to send spam.