[chrisked]

Agree that spam filtering out of the box could be improved. If you want advanced control then write a SIEVE script https://www.fastmail.help/hc/en-us/articles/360060591373-How...

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.