My issue was: I’d set up mailing from a contact form (in my Wordpress/Elementor site) using the WP SMTP Mail plugin. Then I installed Cloudlfare APO and mailing was killed stone dead. Wooaaaa! That must mean that Cloudflare is the problem.
I spent a long time trying to get around this issue, then eventually gave up and tried a different Mail plugin. Success!
I chose Post SMTP. For this you have to set up an account elsewhere and get an API key. The first one offered is paid, I went for the second one, SendGrid.
If this sounds like an advertorial, well, it isn’t. The issue is with WP SMTP, not Cloudflare. Post SMTP is much more easy to use and set up, and you’re spared constant “you need Pro” reminders. Good luck!