Is Fastmail in any way similar to what is being described here? Fastmail looks like a replacement for Gmail or maybe Gsuite.
Sorry... I though Cloudflare was offering full service email (SMTP/MTA). If it is just SMTP outbound email, then SMTP2Go would be a better alternative.
Fastmail is mentioned on every email provider suggestion thread on HN (Because they are great, happy user!), but they are not a transactional email provider which is what this product is about.
By transactional, do you mean a bulk sender? For that, I recommend SMTP2Go.