CiviMail not delivering to certain mail servers eg. wp.pl
Demerit is correct, but I'll add a bit more detail and how you can fix it.

Your CiviCRM sends emails as bounces.stp@gmail.com from 212.77.101.4. The SPF records looked up are those for the sender's domain, gmail.com, which does not include your webserver's IP.

Gmail treats this as a softfail, but nazwa.pl seems configured to treat failed SPF more strictly. Both are SPF fails.

Unless you control @gmail.com, you can't fix this via SPF for that originating email. What you can do is send messages from your own domain, with working SPF configuration.

To address this, change the originating email address (CiviCRM Menu > Administer > CiviMail > From Email Addresses) to eg bounces@stpuk-org.pl. 

This mailbox can in turn forward to your Gmail address, so CiviCRM can continue monitoring that mailbox for bounces.