I think your email provider is doing something wrong or misunderstood something here.
Cloudflare IP is how visitor get to your website. It has nothing to do with your email.
The IP address that an email provider check for blacklists are the origin IP of your own server which from there, you open a connection to the target mail server. How do you know this address? it’s in the maillog, or you can SSH into your own server and run
curl ifconfig.co to find out your outgoing IP. That’s the IP address the target mail server will see, and check for blacklist matched.
Example: my site is behind cloudflare: https://hanami.run and I sent email out just fine even though I used Cloudflare and share the same set of IP address with thousdand of people.
Also, make sure you IP server address(not cloudflare one) resolve to a DNS, and that DNS is resolved to your IP. Example.
Say, your server IP address is 220.127.116.11. Then you have to set a reverse DNS to point 18.104.22.168 to my-server.domain.com and again, my-server.domain.com has to resolved to 22.214.171.124.