Probably JMAP support.

Already a part of the offering. :)

JMAP is a bonus (other fastmail user here) bjt if I can do custom sieve rules and or unlimited aliases created on demand at (somestring)@customdomainiown.com thst I can then use the sieve rules to put into a folder of the same name as that email address, I would rather give my money to support Thunderbird. Fastmail is fine and all but they are in australia so they live on spyware island and they dont have good native clients in the works like thunderbird does.

Unlimited aliases at custom domains are a part of the offering. Technically, Thundermail supports sieve rules, we do need to come up with UX to expose it to users for management.

The few times I've needed support, Fastmail has responded nearly immediately with the exact info I needed. So Moz would need to demonstrate excellence in customer service before I would consider any migration.

(I know that doesn't directly answer your question, but it does articulate a necessary pre-condition, and one that is hard for businesses entering a new line of service to deliver--although not impossible, of course.)