Reverse DNS problem. Any way to force-use IPV4?

Hello!

Recently, emails I send from my virtualmin-managed website (debian squeeze, webmin, virtualmin, default settings as in postfix) to gmail users have been rejected by Google, with this message: (I edited away true IPs and domains)

host gmail-smtp-in.l.google.com[2a00:1450:400c:c09::1b] said: 550-5.7.1 [x:x:x:x::1b] The IP address sending this message does not 550-5.7.1 have a PTR record setup. As a policy, Gmail does not accept messages 550-5.7.1 from IPs with missing PTR records

I checked with my DNS settings (I'm with cloudflare, so it's with them that I edit my DNS records rather than with the web host for my dedicated server) and, indeed, I only had this:

Thing is: it's been three years I've had the very same configuration and, until very recently, I didn't have problems sending emails to gmail addresses. It's as if, suddenly, Google reverse-DNS-reads my domain's IPV6 instead of my domain's IPV4.

So, what I did, was changing the DNS records I have at cloudflare, in which I added my domain's IPV6 address, as mentioned by Virtualmin when I choose my domain:

But sadly, I'm still getting the same rejection emails from Google, my addition of my website's IPV6 address didn't do anything.

Please, would you know if there's a way to FORCE the usage of IPV4 rather than IPV6 in the whole process? Or else, how I may have a slim hope of fixing it? I'm all ears, really, if you can help, THANK YOU!!