After you setup the mail gateway you can telnet to port 25 and do the command line mail test and see if mail is delivered to your mail server.
After that is confirmed you change your dns mx records.
It depends a bit on your needs.
My proxmox setup is like multiple nodes (computers) with local (2 drives with ZFS mirrorig), they all use a truenas server as NFS host for data storage.
For some things I use conaitners (LXC) but other thing I use VMs.
It is, I looked at several vendor's and it's all te same except for the nice gui.
They all have their own blacklist that they feed with the spam/ham queries from their devices.
Personally not, I primarily use Samsung but hear from colleagues that they had issues with other brands.
The other thing is that there are only 2 brands that make screens, Samsung and LG. Those screens are used by all the other brands as well.
They don't have to cost money.
The mail filter appliances are all based on postfix, spamassassin and a virus scanner like clamav. The thing you pay for is the nice gui.
Car manufacturers came to the same conclusion and tada there is Stellantis and 50% of all cars came from the same factory.
Houses are pre-fab for decades.
Humans love brands but many items come from the same factory just with another label.
That there are so many Linux distro's is why it's still a niche on desktops and it will always stay that way.
Microsoft and Google are the company with the highest number of contributions to the Open Source community, that is in volume.
The problem with the diversity is that, because they all know it better, there isn't any perfect distro. If they would work together and fix all the flaws that make people choose another distro it would be perfect and compatitive, now it's just not.
And how much of that range do you use per day?