Are the messages which are being bounced: (a) sent by you, and (b) sent using Fastmail's servers? Or are you using another company's servers on the outgoing messages? Fastmail knows which messages it sent on your behalf, and if it gets a bounce on a message it did not send itself, it will normally put it in spam.
There is a setting under Advanced spam settings to tell Fastmail the other servers you might use to send your email, for example if gmail sends on your behalf.
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