If your VPS host (AWS, OVH, Vultr, Bluehost, Digital Ocean et al) is blocking port 25 and this is causing problems with your Virtualmin installation and testing, I will be happy to offer you free smarthost / mail relay service for testing or low volume email.
To get started, you will need to send me the IP address of your Virtualmin box (VPS) and the list of domain names which will be authorized to send email. I will then give you the IP address of my Virtualmin box and the SPF and DKIM records that you need to apply to the DNS records of all the domains in your list. You will also have to apply a small change to Postfix, via Virtualmin, and you will be able to send email from your Virtualmin box through my Virtualmin box (which already has RDNS) to anywhere on the net without it getting into the spam folder.
Message me via this forum or Google me, I am very easy to reach.