[cuu508]

In a Cloudflare+GMail setup, what SMTP server would I use for sending?

[notpushkin]

Any third-party one! There’s a bunch of options.

The usual ones like SendGrid or Mailgun are probably not great for personal email, but it could probably work. Personally, I’ve used Duocircle for a similar setup and it worked pretty well: https://www.duocircle.com/email/outbound-smtp