The feature was working normally when i firstly installed the scripted.. Lately i bought many plugins - not sure if related to any, but activation mails not being sent out. Even if i click on admin page to resend the mail, it doesnt get sent.

In the meantime, as admin, i get an email tha new user registered, and also i get mails from contact form

on phperror log i see this:
[10-Feb-2023 17:03:03 UTC] PHP Fatal error:  Uncaught Error: Undefined constant "DB_HOST" in /home/hru33ujxrs7c/public_html/oc-includes/osclass/helpers/hDatabaseInfo.php:41
Stack trace:
#0 /home/hru33ujxrs7c/public_html/oc-includes/osclass/classes/database/DBConnectionClass.php(134): osc_db_host()
#1 /home/hru33ujxrs7c/public_html/oc-includes/osclass/classes/database/DAO.php(79): DBConnectionClass::newInstance()
#2 /home/hru33ujxrs7c/public_html/oc-includes/osclass/model/Preference.php(52): DAO->__construct()
#3 /home/hru33ujxrs7c/public_html/oc-includes/osclass/model/Preference.php(42): Preference->__construct()
#4 /home/hru33ujxrs7c/public_html/oc-includes/osclass/helpers/hPreference.php(1689): Preference::newInstance()
#5 /home/hru33ujxrs7c/public_html/oc-load.php(90): getBoolPreference('osclass_install...')
#6 /home/hru33ujxrs7c/public_html/index.php(25): require_once('/home/hru33ujxr...')
#7 {main}
  thrown in /home/hru33ujxrs7c/public_html/oc-includes/osclass/helpers/hDatabaseInfo.php on line 41

maybe somehow related?

May be.
DB_HOST should be defined in config.php

of course it is (to localhost). How everything else would work on site correctly?

Good question
How so it is undefined.
Anyway it may not matter at all.
Do you have delay when resending activation mail?

when i click to resend the activation mail the page reload within 1-2 sec and i get the message: Activation email sent to one user

Ok. The problem is that the activation mails are send with a different way than the rest mails from system. I checkeed logs and everything. Normally, all the email are getting sent from admin@kalo***.com

For some reason the activations mail getting send to [email protected]***.com

The "regter4325' is my username on my VPS and the server1 is the the hostname of the VPS. So, they are getting everywhere as SPAM.

How can i make the mails works as every other forms on site and getting sent from admin@kalose*.com??

solved by adding a Reverse DNS / Adding TPR to hostname and adding one more dns

Thanks for sharing, so what was actually problem?

Ok. The problem is that the activation mails are send with a different way than the rest mails from system. I checkeed logs and everything. Normally, all the email are getting sent from admin@kalo***.com

For some reason the activations mail getting send to [email protected]***.com

The "regter4325' is my username on my VPS and the server1 is the the hostname of the VPS. So, they are getting everywhere as SPAM.

How can i make the mails works as every other forms on site and getting sent from admin@kalose*.com??

that was the a actual problem. The contact form and the mail sending to admin having as sender the email you setup on osclass. The activation mails were using as center an email "created" from the vps hostname which was a subdomain (something like [email protected]) - so since the SPF dns was setup only for the main domain and not for the subdomains, all email clients were marking it as spam and not delivering it. So i had to create a PTR on the VPS and add also the subdomain on the SPF dns

Thank you!