After Sendgrid got acquired by Twilio and retired their free plan, i also went with Brevo and am quite happy with it. No deliverability issues on the major email operators (Google, MS365) at all, decent delivery speed.
Indeed, but we're talking about transactional/newsletter email providers here, which isn't what Fastmail is for, and beware it's a violation of their terms to use their product for that [1]:
> The Services are not designed for sending automated or transactional emails. It is a violation of these Terms to use the Services to send programmatically generated emails to addresses that are outside your Customer Account.