Its not on FastMail, its on Sieve. It is a hook that runs on receiving. To my knowledge if you want Sieve this is the only option. I use it with Protonmail and it is the same way. I also run it on my own mailserver.
One nice thing, for me at least, is it runs prior to the email ending up in your inbox and I am uses to Outlook rules were rules execute post arrival.
