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.
> 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.
[1]: https://www.fastmail.com/policies/terms-of-service/