Missing from the article: how to communicate progress and failure to the user?

This is much more complicated with task queues. Doable still! But often skipped, because it's tempting to imagine that the backend will just handle the failure by retrying. But there are lots of kinds of failure that can happen.

The recipient's server doesn't accept the email. The recipient's domain name expired. Actually, we don't have an email address for that recipient at all.

The user has seen "got it, will do, don't worry about it" but if that email is time sensitive, they might want to know that it hasn't been sent yet, and maybe they should place a phone call instead.