Noting else works.
I watch tons of videos, noting helps
You will need to un-proxy mail.domainname.com
Or get a 2nd IP just for mail.domainname.com, if you don't want to expose the primary IP.
When the mailserver checks the PTR & rDNS it will fail, as Cloudflare's proxy DNS doesn't have any entries.
Also trying to use Cloudflare's NS servers in an MX will get your IP into trouble, as their system could see it as a spammer attack.
Also if you are using a NAT for a local network, that also makes things more complicated, as your ISP may not allow port 25 access.