https://www.mail-tester.com/

Here I tested the email I sent and complained that the php script is sending the email and learned that I should set up the email to send smtp. How to do it?

Thank you very much, I will discuss it with my service provider.

Well, I would try to isolate things first ...
1. My server was sending an email to your gmail address, and you say it appears correctly in your incoming folder.
So it's basically not a Ossn issue and not a gmail issue.
2. So try to register at your site with the same gmail account of your own and see what happens?
If the mail is appearing in the spam folder this time, than consult your provider. If not find out which email addresses your other recipient with issues were using. So its likely their boxes have configured some filters

No spam, incoming messages in your email. Should I talk to my service provider?

Ok, so you registered at https://comz.z-mans.net
and received my returning mail in your spam folder?