While we're kind of discussing elements of a successful spam defence in a Fastmail account, it is worth mentioning the thermonuclear option. And that would be deleting one's account login username email address.
At the time of my initial sign-up to FM, what I primarily needed was a reliable web-based email service to deliver me from the clutches of email accounts shackled to internet service providers and from the ever widening maw of mammoth info-tech companies and their 'free' email accounts.
After signing up, I chose an appealing top level domain from FM's list, made up a username to go with it, and I was away. To this day the address I created then remains my preferred personal email address.
Much later, I began reading these forums and my awareness of FM's powerful features slowly grew thanks to the tips and tricks revealed here by other users.
One tip that stopped me in my tracks involved deft application of alias addresses in one's account from the get go. At its foundation was the creation of a Fastmail account with a username that
only you know and never reveal to anyone.
That login email address used for your account might have a format like this:
random-character-string@fmdomain.tld
It need not be a string of 23 characters as I have recorded above, and it doesn't necessarily need to be random. The key to this strategy is you never use it to send or receive email to anyone, except messages from Fastmail itself.
After that you use your account to create an appropriate alias, and make this your primary preferred email address for personal correspondence.
The benefit of this strategy is that if for whatever reason the use of your preferred alias email address goes sour, you can kill it without affecting the remainder of your FM account. Also, should someone try to hack into your account they will not know the user login to test for password access.
It is possible to enact this account structure at any stage by changing your account login address (which, from memory, you are allowed to do a limited number of times per year) and then using your old login address (which is probably still your preferred personal email address) as an alias.
Imagine if your preferred email address was getting hit with an insufferable amount of spam and even FM's aggressive spam filtering plus rules created by you couldn't stop it. You could nuke that email address and the spam would cease instantly. Sure, there would be some work to do beyond this to re-establish email contacts with family, friends, business, and the plethora of online entities you are subscribed to. Admittedly that could be a lot of work, but it is possible.
Thankfully I've never had to resort to this brutal use of FM's alias address feature, but it is there. Realistically, though, other account features offer so much control of spam that this problem alone shouldn't be sufficient reason to reach for the launch code cipher book.