mxtoolbox checks and SMTP reverse DNS not a valid hostname

I use ISPconfig and went through today and updated all my domains on the Linode DNS manager to now use SFP, DMARC, DKIM, etc. I think I've massively reduced my spam :)

In the process of checking it's all working OK, I ran some tests using mxtoolbox and for all my domains, it complains that the reverse DNS is not a valid hostname. I presume this is because the reverse DNS points to the real host (xavserver.co.uk), as declared in IPv4 and IPv6 through the Remote Access tab of my Linode and not to the MX record which will be a subdomain, mail. as defined in the DNS manager. Also, there are around a dozen different domains I run on this server through virtualhosts.

Sending emails appears to be OK now - I can send emails to my testing Gmail account. In fact, it was an initial failure to deliver through an IPv6 security issue (IPv6 reverse DNS not initally configured in the remote access tab). Since going through ISPconfig and setting up the various TXT entries, the SFP, DMARC and DKIM all appear OK and email does actually get delivered.

So I'm just worried right now about this warning mxtoolbox comes up with.

Is this something I should worry about?