Log into Fastmail’s Web mail and click on the Mail menu at the top left.
Click on Settings, then Filters & Rules:
Then click Advanced settings:
Make sure that Off is selected:
Make sure that Mark spam as read is unchecked.
You can optionally 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.