During the initial month (the "free" trial) on Fastmail, they make use of an "intensive" spam filtering system that can delay your emails from being sent by days (or not being sent at all).
When contacting support, this is all expected and nothing the user can do.
So essentially, due to this policy, Fastmail's "free trial" is useless, as the mail is neither "fast" nor, in some cases, "deliverable".
I can't speak of its service after the first month, but who* in their right mind would give money to a company that has such an unreliable delivery of emails (even if temporarily) ?
* here, by "who" I mean new customers, not existing ones that "never had a problem", of course