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