Can I use Cloudflare for my Linode which runs a website and mailserver?

While you can use Cloudflare to reduce the visibility of your IP address, you are not able to use it to hide the IP address of your MX records. This means that since your MX records will reveal your IP address, the value of using Cloudflare to reduce visibility of your IP address is negated.

When setting up the DNS in CloudFlare, you'll need to "Grey-cloud" your mail DNS records so that mail traffic is not proxied through Cloudflare. If you are using the same Linode to host both your website and mail server, your IP address will become visible as it cannot be proxied through Cloudflare.

I would recommend that you create a separate Linode to host your mail as it will have a different IP address. By doing this you ensure that your websites IP address can still be used with Cloudflare.

For more information from CloudFlare on this topic you can visit the Cloudflare support article below.

https://support.cloudflare.com/hc/en-us/articles/200168876-Email-undeliverable-when-using-Cloudflare