[sph]

It's not. I wrote about this a couple times. Here's how it works: the personal spam filter kicks in after having trained on ~100-200 spam emails. You need to mark them as spam (you might want to use a filter initially if they all have a common pattern) and delete them regularly from the spam folder. Spam training kicks in only when you a spam message is deleted (to avoid training on false positives)

I went through the same phase when I first moved to Fastmail, and this is what customer support explained after some back-and-forth. I don't have spam in my Inbox any more (despite receiving ~20 spam emails per day)

[zaptheimpaler]

Are you talking about false negatives? The problem I have is with false positives. It regularly throws important legit email into spam.

Just the other day it threw an invoice from AWS into spam! Regularly throws financial docs into spam. Like it’s hard for me to imagine how that happens.

[sph]

Training a spam filter works both ways. Mark them as ‘not spam’ until it stops happening.

I never got a legit email end up in my Spam folder, so YMMV.

[brandonwindson]

Fastmail also uses global blocklists and reputation scores that can override your training. Could also be the sending IP is shared with bulk senders.

Marking as "not spam" should help over time. If it does not, create a rule to always deliver from that domain. That is the only way to guarantee it never happens again.