check out the “virtualbox” guide on this site: https://itsfoss.com/install-linux-in-virtualbox/

it basically teaches you how to install/run linux inside a “simulated computer” on your computer, so you dont break anything important

seems to be decent and i couldnt spot any obvious errors in the guide

i usually rely on https://mxtoolbox.com/ for quick checks good overview about mail and dns related issues

fuck, half of my critical services think I am spam

that goes away after your dns gets older and your mailserver gets “reputation”… also configure your dmarc and dkim, that tends to help a lot

another reason could be that your public ip is “dirty”, check the reputation of it and maybe request a different one from your hoster

i see it more like telling somebody to weightlift 50kg first instead of immediately going for 500kg

then you might be in over your head and shouldnt use what you are not able to use