When I was testing this I created an account in claws-mail that sends through FM's server using an Fastmail alias without an FM identity. This will go out using the low-reputation forward* servers.
I've just started a trial of Namecheap's email hosting. Today I was testing the account's filtering rules and I used the above claws-mail account to send test emails (forgetting why I created it).
Everything sent with that claws-mail account is being marked as spam by
SpamExperts and ending-up in the spam folder. Mail that goes through the out* servers is not.