When first registering on this forum I had the problem that the registration e-mail did not arrive.
It turned out the reason was that it was rejected by the incoming mail server, due to failing SPF checks.
Only after white-listing your registration mailer’s domain and IPs (both IPv4 and IPv6 was necessary), ignoring SPF checks here, the mail did arrive in my inbox.
As others mentioned before already here in other topics (no links because I am only allowed to include two in total) the registration mail is marked as spam by Outlook as well. And I assume it is probable that it will be rejected by many more.
After testing your registration mail with the tool at mail-tester.com by registering a test account with a generated test e-mail address from there, the result says: “Consider yourself lucky if your email goes to an inbox”. (Score 4.8 / 10)
More detailed test results I pasted here: https://justpaste.it/71bac
The most serious is probably the failing SPF check, and a suggestion how to fix it is included in the error report. But I think you would also have to add the IPv6 address, because the first time it failed on me the mail came via IPv6 and I had to whitelist both your mail server’s IPv4 and IPv6 address on my end before it arrived.