FastMail is absolutely flawless. Their feature improvements are always on point as well.
I love Fastmail for my personal email but the spam filtering leaves a lot to be desired. For my startup we tried to use it but had to move because of deliverability issues.
Deliverability issue are foreign to me. Never had an issue in over a decade. Did you configure everything properly? Also, ask their support about this with a concrete example. They’re very helpful.