Finally got my email server set up, thank to @vasileios 's excellent LMS tutorial. I am starting to understand the DNS records.
On of the steps was to do a DNS checkup (Domain DNS Health Checker - Check DNS, Blacklist, Email, MX Health), the only warning that was not in the video was the “Reverse DNS does not match SMTP Banner”.
What does it mean? And if I need to fix it, how?
Hey @Ai!
If that warning is all you’re getting, then you can let it be. It essentially means nothing and I’ve seen many bigger email providers getting that warning as well. The moment you get a high score on the mail-tester.com, you have nothing to worry about! 
@vasileios, thank you for the answer! I have 8/10 from mail-tester.com, so that is good.
When I was testing sending and receiving for the new email account with another one hosted by a service, the emails to that account showed up as spam (by SpamAssessin), I understand I can whitelist it on that end for myself but what about people that I will be writing to?
Will this problem go away as my domain name “grows” older?
Hey @Ai!
You are very correct in your assessment. As you continue to utilize your email in a legitimate way, the more your score will increase. Speaking of which, what is the main justification SpamAssassin gives you when it characterizes your message as spam?
Essentially, you may undergo a process where your email may end up in the spam folder of some of the people you are writing to. This is due to some extremely strict blacklists, who resort to blaming an entire company rather than the individual users (which is - please excuse my language - BS). I had to deal with one in Switzerland, which does this blacklisting. The excuse they offered was abysmal. My mail-tester score is over 8.0, but they still block it. Not for me, individually, but for the entire company I run my mail server on (in this case, Linode). But then, they go as far as to say “pay us per month to white-list your address” - which is obviously extortion.
Due to that, all my emails to iCloud accounts never go through at all. At the same time, they are able to reach other Big-Tech platforms with ease, and go directly into the inbox.
Is the spam classification is something I can check myself in my cPanel Or do I have to ask my host?
I am surprised to hear about your experience, I wonder if that is also the reason why I never get any email from mobile.xyz even after I’ve put it on my whitelist…
Hey @Ai!
You’ll have to check if your domain or IP address is listed on any of the blacklists. Those are responsible for emails that become undeliverable or end up in the spam folder.
If you go to MXtoolbox, you can input your domain and see if you are listed anywhere.
When it comes to your provider, is it a hosting package or an email provider? If it’s the former, you might want to see if the IP you have is listed in any of the black lists too.
Here’s the link to check your domain and/or IP:
Got 1 blacklisted out of 91 lists. I don’t plan on doing newsletter with this site so I guess I am okay. Thank you for the link and answer! 
We’re on the same boat, then! I got one of them (the one who wants money) blacklisted me too. 