The Fastmail article linked my Mr Duggan [0] is also worth a read, they provide a sober and reasonable overview of why they don't offer PGP. Of course, Australia has a quietly privacy-phobic regulatory regime so we can guarantee [1] that Five Eyes countries and possibly others are reading emails sent through Fastmail. Cost of doing business really, I use Fastmail.
Fact is that there isn't a way to use a convenient 3rd party email provider if you want secure emails. Only local clients can provide that feature - which means both sides of the message have to be using trusted local clients, and at some point one side of the conversation will forget their secret and lose access to their email history. It is a tough problem.