What is the name of the domain?
What is the issue you’re encountering
This record exposes the IP address used in the CNAME record on mail.buypackagingboxes.co.uk, which you have proxied through Cloudflare.
Welcome to the Cloudflare Community. 
Let’s address the largest issue first: Using a CNAME as the target of an MX is an invalid practice. You must use a hostname that has A or AAAA records.
Using proxied host, like you are is also an invalid practice. Without an Enterprise plan with Cloudflare Spectrum, the Cloudflare proxy relays HTTP and HTTPS traffic only. Cloudflare attempts to compensate for the invalid configuration by using a synthetic hostname beginning with _dc-mx., but hostnames that begin with an underscore are invalid and this will invariably cause problems. You need to ensure that any hostname in your MX is set to 
DNS Only.
Finally, concerning the original question you presented, operating your mailserver on the same host as your webserver will always result in the warning about exposing its IP address. The only way to avoid that is to move your mailserver to another host.
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