To continue on this topic Hosting my/your Email Server and hopefully help some Cloudron users understand the reason of this, here's a little story.
After building your self-hosted server, Cloudron and/or any self-hosted solution, you run all these test successfully and to be confident that your emails will reach their destination,
- Reverse DNS
Even Mail-Tester give you a thumb-up rainbow (10/10).
But then you send you email to your friend, or worst, your client and his using Microsoft (Office365) or Google (gSuite) as the mail service to receive. If you're lucky, you email passes their inHouse Spam filter and your friend/customer receive your email. With less luck, the message goes straight to their SPAM mailbox or, even worst simply don't reach them at all.
Some ConspiraZionist would say: they want to control the world and kill all small businesses,....
While this may be true, the fact is that Microsoft and Google use their own private Blacklists (RBL) and they seem to not whitelisting IP as often than SpamHaus and/or Barracuda which are public Blacklist.
Another reason might be they enlarge the range of the subnet instead of only declaring one IP at time, such as an example if you are hosting your server at Hetzner and your neighbor with the IP 188.8.131.52/32 is marked as a spammer they might ban the all range of 184.108.40.206/24 or even worst 220.127.116.11/16 which represent a big part of their entire network Hetzner AS.
They can also use a set of different pattern which could vary from how long is the email, how similar it look like a phishing attempt, ...
Unfortunately, Microsoft and Google don't really provide support and guideline for this kind of situation. Google has it's postmaster.google.com but even if your domain is verified,... you have to send a s**t load of email per month to be considered a legitimate sender.
That's one reason why Cloudron support Mail Relay such as SendGrid and MailJet, because Microsoft prefers to analyze your email at the source and read them before the recipient, who knows, it could be a threat to U.S. national security,... but these e-mails are also very useful for profiling, and targeting and also feeding the Artificial Intelligence
To counter the last part, I suggest that you encrypt your email with GPG or PGP and/or if you have something very important to say to someone, just invite him/her for a coffee or a beer.