I have found Let's Debug very helpful when diagnosing this situation. In your case, unfortunately, all it does is indicate that your site is behind Cloudflare. If you didn't exempt the ACME challenge path from SSL redirection, you can run into issues.
I find it helpful to create a Page Rule for the following:
Allow ACME challenge
*example.com/.well-known/acme-challenge/*
Disable Security
SSL: Off
Cache Level: Bypass
Disable Performance
You also have your A record for your MX proxied. Cloudflare does not proxy any non HTTP protocols, which means POP/IMAP will definitely fail, and you may encounter SMTP difficulties as well. You can see from the synthetic MX record _dc-mx.3fa3f8e858f9.instatech.network that Cloudflare is trying provide you a workaround, but the consensus in the Cloudflare Community is to set any entries that require non-HTTP use to DNS Only.
When I am setting up the initial Let's Encrypt certificate for a host behind Cloudflare, I do that with the relevant hostsnames set to DNS Only, and only after after the Let's Encrypt certificate is working as expected on the origin site do I enable the Cloudflare proxy.